This is also a result of niche erosion. When one character can do it all it becomes far more beneficial to optimize the hell out of that one character rather than to seek optimization of the party as a whole.
With strong and inviolate niches, the party are somewhat forced to cooperate in order to cover off each other's weak points.
The niches don't need to be inviolate. They just need to require effort to build beyond where you started, so that you can only grow into new roles slowly over time.
That is quite literally what 4e did. Its roles were
not inviolate. Paladins could
become solid Leaders. Fighters could grow into doing rather impressive amounts of damage, almost like a Striker. Avengers could have insanely tanky defenses and, if they got a way to apply Marks, could actually pull off Defender stuff despite being "robe" wearers. Etc.
It is not being inviolate which makes the difference: it is the
synergy which makes the difference.
The game must be designed so that the optimal thing to do is either to be one who sets others up for success but can't do the follow-through personally, or to be one who can follow through on someone else's setup but can't personally do the setting-up. It requires that "support" focused play actually be entertaining and engaging in and of itself, which means that "Brother Bactine" must become a purely opt-in choice if he's included at all. Instead, buffs and coordination/deployment become the primary benefits of support-designed classes.
As long as D&D magic remains as powerful, diverse, and necessary as it is in 5e (and as it was in 3e), you won't see teamwork be the focus of optimization. The absolute best you'll see is stuff like Treantmonk's "god wizard" guide, where other classes are viewed purely instrumentally, and the Wizard
permits them to feel useful while knowing that their power is what actually solves the problem, everyone else is on cleanup duty.
Unless and until the game makes it more valuable to care about how you can make the
team better, rather than purely focusing on personal optimization, people will remain focused on personal optimization. Design that demands synergy in order to achieve the best results is design that naturally fosters teamwork.