Ah. Right, I forgot that it was called a strike. Dumb me. Current Mihoyo's voice actors' strikes make me thought something else.
The ToS have been changed here and there are various reasons why other groups, regardless of whether their series has been taken down or not, have opted to leave through fear of legal action. Do a little digging and see for yourself (or DM me and I'll explain). It's 100% not as clear cut as "This series didn't get removed, we'll just stay".
Well, we all know that nothing is really safe. Just need one decision, either from the author or from publisher, and a series will be take down anyway.
I think it is still good to use Mangadex as some kind of hub information. So if a group take a series, we know what they do, and where we can find it outside Mangadex. There is Manga Updates for that kind of thing, but the UI isn't really supporting the UX.