I think what people don’t want is the audience and culture that Threads is likely to bring to the fediverse, not so much Meta itself.
The audience is not the problem. Meta’s mere presence on the network will be. We are now at a critical point in the struggle to survive as a network, and it’s not looking good.
If we continue like today, the network effect (Google it) would eventually lead to ActivityPub being the de facto too-big-to-fail standard in all of the web. We aren’t there yet, though. Meta knows this too and doesn’t want it to happen, because extracting value from a diverse network is way harder than from a centralized user base. The fact that they even want to federate in the first place (shouldn’t be in their interest!) rings alarm bells.
Honestly, there isn’t much else we can do. Spread the word that there are better alternatives to Threads and don’t let them join us. If you prevent “If you can’t beat them, join them.” then that’s a step in the right direction (survival of the network).