This is what I'm fundamentally arguing against though. The card (or cards) you play don't matter. Why does 6 mana mean game over for aggressive strategies? When did that become a design principle of MTG? It's simply not part of the original game. This is a new concept that has been introduced to the game in the last 8 or 9 years.
Turns 2-5 should really determine the winner of most games. IMO at least. If you are the beatdown and you have established a really strong board presence and gotten into a winning position, there should be no card or combination of cards that cost 6 mana which should invalidate what happened on T2-T5.
I realize that many (most?) cube environments this simply isn't true, but that is the very crux of my argument.
I've played games where my opponent simply got a whole lot of momentum on me that by the time I got to that TC for U, it could have read "draw your entire library" and I still probably would have lost. That's an extreme scenario of course. Often times I have a card or two in my deck that could turn the tides, but this is where the 7 card dig on DTT is better than getting just 3 cards off the top from TC. And you pay for that with UU versus U.
Both DTT and TC are very powerful cards. Not arguing against that. They are also cards that very often pull me into blue. I can understand not wanting that effect in your cube too though.
err...I don't know if thats ever really been true. At the least I think its a lot more nuanced than focusing on 6 mana ETB monsters or planeswalkers. TC would be just as broken in 1994 as it is today, for the exact same reasons: it puts you ahead on both card advantage and tempo. That was always the strength of the card: the way it snowballed card advantage into board position, because you could cast the cards you found the turn you found them. That was the same power behind
ancestral recall, and being slightly more constricted in the way you can leverage it, turns out, does little to make it less awesome.
TC is kind of an interesting card in the sense it provides some good lessons in pitfalls of card design, as well as card evaluation. We know from the modern pro tour debacle that WOTC's testing is a bit inbred, in the sense that it focuses exclusively on standard, presuming that the natural high power of eternal formats will correct for any mistakes they make.
They were spectacularly wrong with both TC and DTT, as evidenced by the fact that those cards were hideously broken in literally every other single constructed format they were legal in, excepting for the one they were explicitly designed to be balanced for, and that reality should hang over the decision to run either card.
I don't want to come across as dog piling, but I feel that the defense of TC you are providing is almost satirical; as probably a very similar conversation occurred at some point during TC's development cycle, when people began to question whether it might be OP in eternal formats.
I also think your evaluation of the card is almost certainly off, even within the context of your own format.
People were rather slow to warm to it in pauper as well, making similar arguments, starting it out in U/R or UB decks designed to generate gy density, and it slowly crept into mono U delver, maybe supported by
thought scours, before people realized that it was just a good stuff card, that it needed no support, and that you were insane not to just run straight mono U delver with four copies.
Gush, a
free draw spell, couldn't compete in the
tempo deck for space, and never in its history could justify the full four slots that TC did easily with no support. Think about that.
That was a conclusion that took hundreds of people several months to slowly arrive at in a competitive setting. When I see a lone cube designer defending the card, it seems more probable to me that this is more the result of limited reps with the card due to how hard it is to format test (which lets admit, is a huge problem in terms of balancing we all face: we don't have dedicated test groups), and casual playgroups that will tolerate all sorts of format inbalance as long as they are having fun.