I think E/E/E is still a risk. If some "high follower" type people start joining Threads, and people on Mastodon start following them and making that content a big part of their feed, those people are not going to be happy if Threads accounts suddenly disappear because Meta make arbitrary, incompatible changes.
Hopefully it won't actually extinguish Mastodon/the Fediverse, but it can still do damage.