I think T5 is really one of the initial gateways into the real meat of the next level of teamwork in this game. Prior to T5, people could often slack off or not really pay attention and the rest of the party usually had to pick up the slack. From T5 onwards into the new Coil and new Extremes, the real name of the game is self-awareness in every fight. You can't afford to just fluff about and hope that the tanks/healers will pick you up when you stumble or when you're giving it half-hearted attention. To be successful in T5 and on, it's on you to be on top of everything around you: to know the patterns to look out for, to not stand in stuff you shouldn't be, and how to most effectively play your class in those situations. And not only that, but you need to be aware of where others are, so you don't accidentally mess them up through your own actions.

For example, people dying to divebombs often aren't paying attention to when Twintania breaks and triggers that divebomb section. People that die to twisters aren't focus targeting Twintania during the battle to be aware of when she casts it.

I think what's happening is that you're getting teammates that (a) either don't care about knowing the fight because they think they can just lean on someone else to get them through it, or (b) they aren't paying attention to everything around them and have tunnel vision when they fight. You really can't have weak links in a party with the newer content... I think that's what will hold people back until SE artifically lowers the bar for them to enter in later.