Completely overhauled affixes, gearing, rewards, and key depletion.
Personally, I wouldn't claim that they haven't done anything to the M+ system. But what they have done doesn't fix the problems people have with the system:
They have overhauled affixes multiple times since Legion, and phased seasonal affixed in and out.
Gearing is still not intuitive, and the upgrade system overhauls haven't helped much. Most players resent this currency A + currency B bullshit. Also, for optimal trinkets you're forced to raid, so M+ is not a complete gearing solution, because Ion needs his raiders.
And third, the key depletion "fix" was purely aimed at the high end pushers (less than 5% of the M+ playerbase, which is a fraction of the playerbase), and has deteriorated into somewhat of a shitshow when players started selling their resilient keys.
To this day I don't understand why keys at any level don't have at least three charges, and that timing a key at a higher level improves the keys for everyone in the party that has a lower key +1.
While we're at it, Blizzard could improve the group finder system for M+ dungeons.
The optimal solution of course would be to make it more like the Fractals in GW2, but I guess they decided to only steal dragon flying from GW2 instead.
The discourse is around concerns the highest level players have instead of what actually impacts most players. The healing comment is directly from mythic raiders and key pushers.
Yeah, because the larger playerbase doesn't engage in high end content. At all. But for the players enjoying that content without pushing high keys: I can assure you the groups I was in felt it very much. I have never hated healing more than in TWW, and couldn't even be bothered to make 3k RIO this season.
Also, If this was true, then M+ numbers for season 1 of TWW would've been better, but instead they were steadily below DF season 3. Most infographics compare it to DF season 4, but that was basically an extra season when the expansion was already over, and at least half the people already had checked out.