I hope this can be a polite argument of different opinions.
Like others said it could be bad for the fediverse in the long run. If meta joins activitypub they are the only ones that are really winning. We get access and engage their content, which promotes their network. They would be the largest node by far and it will give them power to influence activitypub. They can push “features” that only works within the threads network and when they don’t work with the rest of the fediverse creating a disparity in the userbase. People on threads will think that we are the ones that are weird for not having them since all users on threads have them and will probably push, “just download threads”. (Kind of how apple controls iMessage and how people get bullied into buying iphones just so they don’t have a green bubble, the fediverse is the different one that will need to adapt to meta wishes) Or maybe they can suddenly decide to defederate, and now all the people that had connections with threads will be forced to download threads if they want to keep those connections. People that would otherwise never had downloaded threads in the first place. Regardless of the outcome, they join to stay or leave, they are the ones that will win in the end.
This has been done in the past. Its Microsoft " embrace, extend, extinguish" philosophy. A recent example of an open standard is the XMPP being killed by Google. Ultimately meta is a for profit corporation and they have every incentive to monopolize this space.
You brought up the point of people being able to block the domain. The vast majority of people don’t change the default settings so the fediverse experience would include threads by default. Just like how ppl can block meta, and since the majority of fediverse don’t really want the connection, if threads is so important for you why can’t you sign up on their pratform?
For me, other than my concerns with the future of the fediverse, i also consider is that meta is so bad that threads is not even available in Europe because of safety and privacy concerns and so I want nothing to do with it.
One last thing, meta was supposed to join WhatsApp, FB messages and Instagram direct, meaning from each of those platforms you could message ppl on the other platforms. They haven’t even been able to do that yet. They connected FB and Instagram but not WhatsApp yet bc ppl opinion of FB is low enough and the backlash was big enough.
Hopefully I managed to convey my reasons why the federation with meta is bad.
But also, what are the good reasons to federate, like you (just opposite) I haven’t seen a valid reason to federate.