There is no peril there. There are powergamers, but they are well less dangerous than in 3e for example, as there is much less difference between an optimised character and a non-optimised one. Second, it's not 5e play, it's 5e play with bells and whistles on and in particular feats (and very often multiclassing). Without these, the power gap is noticeably reduced between powergamers and casuals (remove the feat and I'm pretty sure the Rune Knight will be WAY LESS impressive). So please don't complain about what are oveall rather small imbalances when using options which are increase the imbalance in the game. And the final point is that, in any case, the game is not designed to be balanced. This was tried once and did not please, for reasons that can be discussed at length elsewhere, but the designers made it clear, putting too much balance results in a constrained game, contrary to the open-endedness of the game (you don't have to agree, but these people are the professional designers that designed the most successfull TTRPG ever by a huge margin, so, in addition to my personal feelings, I would tend to agree with them more than with the average internet Joe). So yes, as in most TTRPGs out there (although most don't care about balance, it's really that boardgamy thing quite specific to D&D), the balance is squarely within the hands of the DM, and he has all the tools to correct that, whether it's the situations that he puts in play, the types of adversaries, their tactics, what the party finds in terms of items.
One guy tried this at our tables in Tomb of Annihilation (we have a few powergamers left in hiding), but even with the care that he put (Goliath Fighter Barbarian with feats) and the fact that options were allowed, he ended up not dominating anything in particular. DMs are there for a reason, although I agree that the other way around, some DMs, usually unconsciously, create situations that are so similar that some builds are more effective, or let the player twist the situation so that he is more effective. Simply don't do this, or even better make sure to create situations where the power gap is less noticeable.