I think Hearthstone offers a helpful point of comparison. The "Druid" class identity, after ramp, was modal spells. This meant that in the game's first few years, a critical mass of really strong modal spells appeared for the class and codified themselves as the auto-include core of any competitive deck (it's been, god, 7 years, but off the top of my head: 2 innervate 2 wild growth 1 and then eventually 2 nourish 2 swipe 2 wrath, 2 Ancient of War, eventually 2 Ultimate Infestation, early on 2 Druid of the Claw, Cenarius and then eventually the Death Knight, so on and so on and so on....). Eventually Blizzard gave Druid other competitive toolsets that represented different cores of a competitive deck - Malygos, Yogg-Saron, and above all else, Jade - but the "core" of any "real" Druid deck changed remarkably little over many many years.
Each individual 'Choose One' card wasn't a threat to deck diversity, but the problem was created by a critical mass, where they represented a sufficiently branching decision tree to cover (and dominate) any gamestate that might come to pass.
Modal spells are awesome in Limited because generally in retail limited players are looking at - and evaluating between - maybe two or three actually compelling lines or options. In Cube and Constructed, the number of viable lines of play for a given hand and deck is much higher; there is still usually some kind of pretty easy-to-perform heuristic for Hero to narrow her options down to two that she can genuinely weigh against each other. If every card in your deck were modal, the level of analysis required to make really marginal choices ('do I keep Nourish for cards, or slam ramp and hope to draw into UI?' is only one choice for one card but itself already represents a real cognitive load) probably quickly outpaces the actual improvements to gameplay that you're creating by consistency and reduced odds of 'the Jund problem' (of drawing the wrong half of your deck at the wrong time).
what does this mean for cube? probably not much because we're not gonna get that overwhelming majority of all of our cards presenting genuinely distinct modal choices game after game after game. What does it mean for Standard? Has there been a standard format ruled by modal spells? ultimately, i guess planeswalkers mean that the answer is 'yes', but Magic has wisely stayed away from the worst excesses of 2010s Druid.dec. That said, I still doubt any of us are at risk of doing that to our formats. cheers