Well, there are three factors here.
One, Mearls spent a VERY long time blaming nearly all of D&D's problems on stuff 4e did. So to....reinvent the wheel, after 15+ years of being at the helm and treating 4e like rotten flaming garbage, isn't exactly a great look. You're correct that it is good to recognize your mistakes. It's just that that isn't what this appears to be, at least at a superficial glance.
Two, perhaps you are not familiar with some of the things that happened along the road to 5e's publication. One of those things was Mr. Dancey's article where he floated the idea "What about what I call 'passive perception'?" Except...there was a game that had invented that term, and used it extensively. That game was 4th edition D&D. So, a lot of 4e fans are kinda sensitive about designers going on about having "fixed" some problem or "found" some solution....when that solution is functionally just D&D 4e. And you'll note that Mearls does not, even once, mention 4e--even though he's essentially reinvented 4e powers right from the jump.
Three, it seems clear that Mearls' primary goal is to leave the Wizard's total power functionally untouched, which likely isn't going to sit well with 4e fans, since there's still quite a gap between an optimized Wizard and an optimized Fighter, even if we presume that his proposal worked perfectly as proposed. That opens up reasonable questions about whether he has in fact "realized his mistakes", or is just pivoting from focusing on one mistake to focusing on a different mistake (at least in some folks' eyes; obviously there are