I'm not following you here.
Mike Mearls: If a PC being too good at combat is messing up your campaign, the issue might be that your campaign is too combat driven.Disagree strongly. ... Combat is one of the 3 major pillars of 5e.
So if combat is one of the three pillars that means Mc UberGod dominating combat is only dominating 33.3% of the game. Well, actually significantly less really. Since Adventures allow up to seven players, even if Mc UberGod deals four times as much damage as everyone else he only makes up 40% of the party's total damage. His total share & contribution to the campaign's advancement is an equivalent of 13.3%.
Assuming everything was split up, each person is expected to have a median value share of about 14.25% (100/7) with some scoring higher in some pillars at the cost of not performing well in others (aka balance). If anything, your three pillar approach is exactly what Mearls is talking about and the existence of those inherently lessens the impact of someone being better in combat than the rest of his follow players. A character must also focus on other things, otherwise he'll end up being subpar.
This is why Wizards in 3rd were ultimately pegged as broken. It's not that they can deal more damage than an ubercharger, but in their downtime they create minions, castles, items, cure cancer, solve world hunger, and turn turtles into ninjas. It doesn't cost them anything to temporarily master any given pillar.
Well, it's because a 40% damage increase isn't the problem.
Stockpiling spell slots to unlimited quantities during downtime is the most blatant current problem.
Here's an example: a so-called Coffeelock at character level 12 can stockpile roughly 50 level 5 slots every 10 days, increasing the stockpiled slots at that rate without limit so long as downtime persists.
This isn't at the level of 3.x Wizards -- I suspect nothing else is at that level -- but it's in the same direction, and it's for the same reason that you bring up as if it were a distinction.
If downtime exists in a game, there's currently a character who can stockpile resources without limit.
Those resources are not limited to combat, of course. The so-called Coffeelock could stockpile slots for a season, and then keep nine separate level 5 Animate Dead
spells active for 10 days. Quantity has a quality all its own, and all that, and what a Coffeelock brings is unbounded quantity -- or plot-bounded, I suppose.
This is not going to be fixed by DMs caring less about combat.
This is the sort of thing that Mearls should be fixing.