This matches my experience as well. As I primarily DM, my problem was magnified by the fact that only a minority of the party enjoyed the optimization/system-mastery aspect of the game - the rest of us were there to enjoy the plot and move the story along. But as they rose in level, the math made it harder and harder for me to challenge the optimizers in combat without wiping out the rest of the party. I don't DM because I enjoy the math (which I do), but because I enjoy the world-building and the story creation that emerges from the playing of the game. That is what is so appealing about Next - bounded accuracy alone means that system mastery should have less of an impact on those players who don't want to pursue it, while still rewarding those who do.
Wait so a Dwarf Wizard that has an AC of 18 and can use spells to grant people attacking them disadvantage and then proceed to out damage any other class is ok? Its not even 'optimizing' its grabbing a race and 1 feat. Something that can happen randomly 25% of the time. Bounded accuracy means every +1 you gain to something is just that more broken. At least in 3E and 4E a +1 wasn't game changing. I'm sorry, but system mastery is amplified in 5E, not minimized.