I have also walked the halls of geeky academia where people wrinkle their noses and call these musicians average (at best) while they themselves have wet dreams of being half as celebrated. I think for many it is jealousy. They would more often than not rate musicians on a scale from Great to Bad that is completely aligned with Great Technique to Bad Technique and funnily also completely reversed from Unsuccessful to Successful. Also see “Wine experts”.
Had more to do with luck, knowing the right people and having the right look than anything to do with drumming ability.
How popular something is, or how much money the artist makes, doesn’t even enter the equation.
People like what they like. Whether other people also like it seems beside the point.
From the artist’s point of view, hopefully they are making the art that they love. Trying to do something because you think it will sell is business, not art.
But is that all there is to it? Should not the drumming serve the song? I don't believe a lot of Metallica's music would be improved by making the drumming more technically impressive (at least for their earlier stuff, I have my gripes about the drumming in their last couple of albums). Their style is simply one which benefits much more from the unique but easy grooves rather than highly technically impressive drumming. And IMO, this simplicity is one of the things which distinguishes them from most other metal bands. I love myself some Slipknot, but I would not love that kind of drumming on Metallica tracks.
If I wanted to claim he's a terrible drummer, I would point to his inability to keep up live, how he has a habit of simplifying parts and missing timings in a way that sounds bad.