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.