If you want to leave Twitter cause it keeps breaking there's a few groups you might fall into:
a) you don't give a damn about openness or tech or any of that but just want something that works
b) your first priorities are "where the people are" and "it works" but federated and open protocols are a big bonus
c) you absolutely want to go somewhere more open instead of another centralized service
Group A is big, and generally folks not on HN. HN itself splits between groups B and C.
If you think group B is a fair bit bigger than group C, but want the open stuff to thrive in the long term, then an initially-"friendly" Meta controlled app can harm you by attracting a big part of the people in group B and then slowly degrading the experience for folks using open clients over time until finally cutting it off. Most of group B won't migrate again at that point as long as they don't fuck up the experience completely.
Whereas if the Meta version wasn't "friendly" at first, much more of that group B might move straight to open things, and then stay there, creating a larger long-term userbase.
It's a way to keep people from fully jumping ship to open solutions by offering short-term openness that will dwindle over time.
If there are more Threads users than non-Threads users then non-Threads instance admins have to choose between adapt or risk emigration. EEE is a consolidation tactic after all.
Many of the people on Mastodon today are perfectly happy without Meta users on it, myself included. If they try to hard-fork, I'm fairly certain the self-hosters would just walk away and let the links/outgoing support break on Meta's app. They need peering networks more than peering networks need them.
They don't have a diverse content-base yet. The Fediverse "shall provide it" to them if it goes according to their plans. Once this deed is done, the Fediverse is no longer valuable to them.
The EU believes social networks and messaging apps should all connect to each other. This is a bad idea (it makes spam filtering impossible) but nevertheless it's in the DMA.
Mastodon admins might find that law applies to them too.
I'd wager Mastodon is ready for this. Abhorrent/illegal/spamming/offensive/offending instances can be defederated from to remove liability at a server-level, and users can self-moderate with "block instance" functionality from their client. Both sides are sufficiently equipped to filter their respective feeds.
It's the realm of pure fantasy to envision a world where the EU bans an original community project to force everyone into a Meta-designed fork. Mastodon users should be safe as long as they're free to run their own server and client software.
That's fine. Blocking their entire website from ever hitting your feed takes 2 clicks.
My point is that this really doesn't matter. If they use ActivityPub as-written, both clients and servers can stop Meta content from reaching their feed. If you just find Threads content annoying, you long-press the post and tap "Block Instance". If you're a site admin and have been given a legitimate reason to block Meta for API abuse (eg. poorly-moderated content, spam, advertisement) then you can exercise your defederation power and be within your right.
Meta has money to pay app developers to develop a good app.
Most ActivityPub services have taken ages to get decent apps and even now Mastodon has some obvious problems. Opening someone else's profile if nobody on your server follows them shows you a barren timeline with no history and there's still no way to tell Mastodon "go fetch toots from this user's outbox".
Meta has lots of money, which the Fediverse does not and needs money to stay online. This is why admins of very large instances signed NDA with Meta to federate with them.
