RDF is useful if the goal is completing content, but not if the goal is interacting with other people, which I believe is a crucial part of the MMO acronym, even when I don't have much time to game. If my goal is to complete content, there are more interesting single player games I can spend my limited amount of game time playing than WoW.

IMO modern WoW is designed to give you the sensation of completing content so rapidly that you mistake the resulting dopamine hits for the feeling of having fun. Meanwhile, anything that could interrupt that cycle of hits has been optimized out, which includes virtually any dependency on another player. (Vanilla has quests that require you to find another player to craft you an item! They never made that "mistake" again...)

I currently run a 10m "dad" guild in WotLK classic. We're only on for 1 night a week for 3h to raid, and virtually every week at least 2 people can't make it due to work, family, or other reasons. And it's fine. Yeah, we progress slower, we still haven't even fully cleared Ulduar which was 2 phases ago, but it makes for a more rewarding experience IMO. The goal isn't completing content, it's interacting with other people.

Meanwhile, when you queue in RDF, no one talks, everyone already knows all the fights, and if you don't keep up you will be vote kicked. I don't see the appeal. TBH I don't even see anything "massively multiplayer" about WoW these days. Everyone else running around could be bots and I wouldn't have any way of knowing. The hardcore WoW servers are probably where the most interesting multiplayer experiences are happening these days.