It doesn't take powergamers to make 5e a cakewalk but they will blow this system wide open. Which is why I finished my 5e campaign and dropped the system. Then again if I run a fantasy game again it will be a meld of skirmish wargaming with some RPG elements. Despite me slamming it as not D&D I may end up in 4e for that reason since it looks like it would fit my desires better.
If you're looking for a fantasy skirmish wargame that doesn't have a great potential for powergaming, then 4E isn't the droid you're looking for.
Coming from someone who spent most of 3.5 and 4E hanging out on the WotC CharOp boards, in 5E your "blowing the system wide open" isn't nearly as big a deal as it was in 3.5 or even 4E... Look at the threads on this forum - there are only a handful of things (more specifically, ways of using those things in combination with others under specific circumstances) that are ever discussed as being "broken", and none of them are anywhere near the level of something you'd find in 4E, nevermind 3.5.
Being that there are so few options, even the moderately squeaky wheels tend to be perceived as squeaking just that much louder.
If you think powergaming/system mastery is a major issue in 5E, I have to wonder if you've ever looked at the CharOp forums of the older editions and seen what "blowing the system wide open" really looks like. In 5E, the disparity between "optimal" characters and un-optimized ones is negligible unless you're looking at it under a
very powerful microscope - there just aren't that many things you can "optimize" since there just aren't that many elements to 5E. It's
designed to encourage playing a single-class character who takes ABIs in preference to all but a handful of feats,
and both multi-classing and feats are
optional systems to begin with.
4E is a hell of a lot crunchier than 5E, and even after all the ridiculous nerfs it suffered there's still more opportunity to optimize any one class in that game than all the classes in all of 5E.
Not to mention,
most of the optimization choices in 4E directly affect that tactical skirmish-game aspect you were looking at the game for.
If a 5E character being able to roll an extra damage die or two or getting an extra attack during combat offends your sensibilities, any 4E character built with even a moderate eye towards combat efficiency will make you cry...