Learning a new route through a dungeon that's never had a m+ variant before is actually interesting and engaging gameplay, doing a raid you've cleared dozens of times isn't particularly fun.
I mean season 4 of Shadowlands will have this with old raids being relevant again. Everyone I know who raids is basically already looking for other games. Nobody wants to do raids that they've already done for months on end. I mean people raided mythic Sanctum for like 8 months, now they want people to go back and do it again. Really don't blame anyone for quitting if that's the type of content they're expected to do.
It works for m+ because the dungeons will feel very different, not really because of the affixes, but just because you'll have to learn new routes and figure out the best way to get your percent and save time. It won't work for raids because it's gonna just be the same thing you've already done.