With Twitter, situation is different since most celebs are still on it and people generally use it just to see what the popular people are saying. Once (and if) these celebrities join Mastodon, Twitter would start to finall fall.

Wow even if admins take over that subreddit, such a big subreddit will surely create a snowball of many more subreddits doing it.

I don’t see reddit ever recovering from this (They’ll not die, but things won’t be the same)

Lemmy and kbin are different things which work on the same backend and are part of the fediverse. Similar to how mastodon is fediverse counterpart of Twitter, lemmy is fediverse counterpart of Reddit, and kbin is a unique thing which is more akin to old school blogging sites.