Once content dependence is established, there is no turning back.
Everyone who is on the fediverse has already made that choice. They are intentionally on a network with less content because of the other benefits And a huge portion of the discussion of Meta joining the fediverse is made up of ppl who are saying they will block Threads on day 1.
the latter will make it impossible to grow again.
That's what everyone was saying back when the fediverse was even smaller, or even before it existed. "How can you compete with giants like G+, twitter, and facebook?" There will always be groups of people who will not participate in corporate social media. And there will always be people who like the convenience of corporate social media but get fed up with it and seek alternatives. And there will always be people who bounce between services.
Tons of people will leave platforms like Mastodon to go to Threads
They're only here because they left corporate social media. If they were going to leave for Threads, why wouldn't they do it now? They've heard all the warnings about some supposed EEE and assume that Threads won't connect to the fediverse forever so why would Threads adding ActivityPub support suddenly change their mind? Going to threads now puts them in the same state as going to threads later in some hypothetical future where the fediverse is too small to matter.
If we want a fediverse with the values we care about to grow...
I don't care about Meta and I'm not relying on them for anything. When they join the fediverse, individual instance owners will still have all the power. User on the fediverse will still be able to control their own feeds. But there are people who use Threads and being able to communicate with them would be nice. I don't think any of the fears about Meta on the fediverse are justified and I think the fediverse will continue on just like it has for more than a decade.
Blakerboy777