As soon as they start pushing features, it’s no longer ActivityPub, but a fork of ActivityPub. There’s no reason why our fedi clients would be forced to adapt. We already have this weird display issue sometimes, like upvotes and threads on Lemmy not properly showing up on Mastodon for example. It’s not a huge issue if it’s not entirely interoperable.
That just download threads mentality already exists. If you think it’s an issue on the internet, it’s 10x more powerful in person. It already exists, it won’t suddenly appear when Meta federates. If you make a new best friend in threads from Mastodon and Threads defederates, surely that isn’t your only point of contact? If it’s that important to you that alternative means of communication isn’t viable, then maybe just download Threads if they aren’t willing to download Mastodon. That’s more of a social issue which greatly varies per person.
I think you’re speaking for others when you say it’s too hard to defederate for users. For the sake of Mastodon, you just press the three dots and block threads.net. That’s very easy UX, no settings involved. Also I can say the same, if choosing to restrict everybody from threads instead of just yourself is so important, why can’t you simply press that block instance button? That way you aren’t taking choice away from others.
The reasons to federate should be obvious. People. That’s the whole point of social media. I don’t want to be restricted to fedi users who think they’re superior then the average person who uses Facebook. I’m not going to stop using Mastodon either just because I don’t like these people. I want to talk to family and friends. I want to invite the people I actually like because decentralization and growing the fedi is good for all. More content the better. That’s what social media is.