1) I think most anti-juggernaut people wanted the fall of the current juggernauts, which is different than saying they don't want any popular pairings to exist whatsoever.

2) If people are really drifting from juggernauts en mass, there's probably a reason. The best writers aren't there anymore, or the fandom's gotten stale and wanky, or one half of the pairing has been accused of a sex crime, or the people being shipped are getting older and settling down, or the fandom is in a transition period between old and new guards. You can't force people to sustain consistent, high-quality output for a particular pairing for some nebulous good of fandom, and you definitely can't make them if that fandom wasn't ever the one they wanted or one they've since outgrown.

3) I think it's generally a positive thing that there are more players and more dynamics being written, and if the fandom's also shrinking, that's likely not the direct result of more people reading and/or writing rarepairs, especially since the most popular teams are still the most popular. I also think the claim that shippers of one rarepair won't cheerlead/support other rarepair shippers is somewhat dubious.