I don't agree with your reasoning here, and I'm going to explain why,
but I want to start by saying that your underlying position (committing to an additional color has to mean something) is totally valid. An undesired "5 color soup" problem plagues many Cubes, and that can often lead to situations where card choices never reach beyond "take the most powerful thing in the pack." However, the reasons you state more fixing could be negative are not normally true in practice. Here is why:
1: More fixing lands makes more cards desirable per draft, not less. There are many cards that are improved by having better fixing. 2-drop Cards with double colored mana requirements, such as
Knight of the White Orchid,
Grim Flayer, and
Sprite Dragon are good examples of this. These cards are all good in their respective archetypes, they can become very hard to play in decks that don't have many ways to meet their mana requirements. The flayer and the dragon both want to be in decks with roughly 50/50 mana splits to make casting them on turn two easier, and the Knight wants to be in a deck that is as close to one color as possible. By improving the fixing of a format, it becomes easier to play these cards under more conditions. Now, an Izzet player leaning heavily on Red can play their
Sprite Dragon and not have to worry about never being able to cast it on time, or worse, having it rot in hand. There are also some three-color cards like
Mantis Rider that are very fun and powerful, but so unbelievably hard to cast without good fixing that they often get left on the chopping block.
More fixing lands also can let players lean a little harder into their splash color without having to worry too much about ruining their mana base. For example, a Selesnya Blink player splashing blue for
Soulherder might choose to also splash a high-synergy card like
Aven Eternal or
Cloudkin Seer that they otherwise couldn't have even considered. Adding that extra bit of fixing could easily open up a ton of unique and interesting options that might not have been available or worthwhile before.
Really the only time that fixing can hurt the playability of some cards is when the power band is extremely broad. For example, if
Dark Confidant is in the same environment as
Sea Snidd, it's likely going to be beneficial for a Blue player to try and splash the insanely powerful Confidant over playing the mediocre Snidd. Snidd is so much worse than Confidant that any manabase inconsistencies caused by splashing would be made up for by the extreme quality of the splash card. In cases like this, it makes sense to reduce fixing quantities in order to help ensure the lower end of the power spectrum actually makes the final cut of some decks. However, it should be noted that this is an issue of power band and not fixing. A good example of this paradigm is Amaz's peasant Cube. Although Amaz's Cube ran only about 41 fixing lands at 600 cards (or about 25 fixing lands per 360 card draft table), the best deck in the format still ended up being a multicolor soup deck dominated by gold power outliers that were better than anything a one or two color deck could muster. Anything short of removing literally all of the fixing likely would not have solved the problem, and even then, there would still be people trying to play 3+ color decks as a means to play all of the best cards.
2: More fixing lands does not make committing to an extra color a less meaningful decision. This may sound a little weird, but let me explain. Expanding from one draft lane into another is always an important decision, because it fundamentally changes what cards a player might be looking to acquire. It requires players to re-think certain factors like their mana curve, deck speed, removal density, and yes, fixing. After all, adding an extra color can trainwreck a draft due to not getting enough cards to make the addition worthwhile. In formats with sparse fixing, however, committing to that extra color is often a non-decision– even if there is a cool card that justifies playing a third or potentially even fourth color to enable, you likely won't be able to get the tools you need to play the card. If you do decide to go all in and try make the thing work, you might end up in a position where you literally can't cast the splash card and have to either abandon it or live with the fact that you will lose some games because your bomb will rot in hand uncast. One of the things I've been noticing in Midnight Hunt limited is that there are a lot of really cool cards that warrant splashing to play, but you really can't in most scenarios without getting lucky and picking up a
Jack-O'-Lantern or an
Evolving Wilds. A lot of the time it feels like the only way to justify playing an extra color for a bomb is by heavily leaning into that third color, requiring filler to flow your way as well. This isn't necessarily undesirable for every format, but it can make drafting feel less dynamic, less interesting, and more restrictive.
3: More fixing lands makes it easier to play fewer colors, too. Something I don't see mentioned too often in the fixing discourse is that more fixing helps to make playing fewer colors easier in addition to playing more colors. As Sigh mentioned, a lot of Cubes play as 2.5 color formats, with most decks being either 2 or 3 colors. This is for several reasons, but some of it actually comes down to fixing density. Alot of Cubes only run 30-40 fixing lands per 360 cards, meaning there are roughly 4 dual lands per color pair, or between 9 and 12 dual lands per three-color combinations. This means that, for every two-color player, they will only have access to 33% of the fixing availible to the three-color players. This can lead to situations where it is actually beneficial for players to add an extra color as a way to increase the amount of fixing availible to them while drafting. However, by adding more fixing, this dynamic can change. If we increase 40 lands to 60, now all of the sudden a two-color player has easier access to the lands they need. Instead of having exactly 4 lands in their colors, they will have 6. This also means that they will be fighting less with three-color players, who will now have access to 18 lands in their colors instead of 12. No single player is every going to need 18 fixing lands, meaning that fixing becomes less of a premium for the three-color drafter. This makes it easier for the two-color drafter to pick up what they need, and it also reduces the amount they are fighting with three-color drafters. Now, everyone has better decks, two colors are more viable, and three color decks trainwreck less often. It's a win-win-win scenario!
One of the things that I have been finding in my Cube is that ever since I increased my fixing density is that more people are playing straight two-color decks than they used to. Common practice used to be that non-aggro players would primarily stick to two colors, but they would have a third "splash" color for gold cards and any power outliers they could pick up. This was because they would have access to more potential fixing, even though it wouldn't all contribute to their base colors. Now, players are more confidently able to stick to two colors because adding a third color is now way less consistent and efficient than having only two colors. The only place where
more colors are played is in aggro decks, which now tend to be synergy-oriented two-color decks as opposed to linear one-color decks. Instead of aggro being "the red deck" or "the white deck," it's now generally along the lines of "Boros burn," "Izzet spells," or "Orzhov taxes." Monocolor aggro decks still exist, but players are no longer shackled to a single color if they want to have a good aggressive deck.
Conclusion
In short, more fixing is good for most formats provided they have a relatively narrow power delta. It increases the value of most spells, the number of viable archetypes, draft flexibility, and unique possibilities. If better fixing is causing problems, that is likely due to power band concerns and not an issue inherent to fixing in of itself. A lot of Cubes would be improved by the addition of extra fixing to help increase the viability of a broader swath of cards in the format. Given the context of this conversation regarding the color difficulty of casting
Kess, Dissident Mage as a barrier to her inclusion, advocating for extra fixing makes perfect sense.
Again, wanting color commitments to mean something is a completely valid design priority, but fixing quality is not as large of a factor in that decision process as your comments may lead one to beleive. The power band and construction of the environment has much more to do with this than raw fixing. If the best strategy in a draft is to take the best cards from each color and try to play them all, people are going to try to do that instead of trying to build more coherent 1-3 color decks. The fixing lands, in this situation, are just making it easier for people to draft an unintended strategy– they exacterbate the problem, but they aren't actually the core issue.
