I haven't defederated Threads on my instance as it has so far not become a problem. They can easily become one if they for example start pushing up ads into the stream.
Most Mastodon users have what they want, they aren't really missing out on anything. Except ads and spying mainly.
As for missing out, I guess it's a matter of perspective. I would assume that right now most Mastodon users have at least one more account on a mainstream corporate social media site - Twitter, Instagram, whatever. What they are missing out from Mastodon, they get on other places.
But if we imagine a scenario where ActivityPub becomes mainstream and all providers start supporting it, there are two totally different experiences.
An experience of a person on a server that federates with everyone could mean having just one account and follow everything from one place. For example Twitter would support ActivityPub so you can follow Twitter users without an account there. You could follow a Youtube channel and have it in your feed. The owner of the channel wouldn't need to create a separate account on Twitter, Instagram, Mastodon, or something - their channel is their followable account on all platforms.
The experience of a person on a small mastodon server that defederates from big corporate servers would be exactly as is today. To follow users of Instagram, Twitter, Threads, Youtube, etc you'd maintain another account. But in the future where ActivityPub is mainstream, that other account is also activitypub-compatible. So, why have two when one is less "powerful" than the other?
Letting the imagination run wild, in such future scenario ActivityPub feeds could be integrated deeply into your iOS/Android phone UI, without needing a separate app. Perhaps also on TV. Most people will want an account that doesn't limit them.
They can also post as their users. Like YouTube, when you look at a video, they put ad videos there in the start and in between, so they could do something like that by posting ads on influencer accounts.
They can also reward the people themselves for posting ads.
Anyhow, I already got one spam message on Mastodon from that browser-embedded instance, Vivaldi, because they didn't do a great job in vetting their users. I was already ready to block them if I receive another, but I never did.
They will find a way to push garbage on ActivityPub I have no doubt.
Regarding defederation, there are different levels. Typically you would silence the instance but allow people to follow single accounts from there.
If it's more powerful to be fully open, then an email client without a spam filter would be superior to one with a spam filter.
Twitter is moving towards closing up their network from users not signed in. EU is making it mandatory for social networks to be interoperable though, so that pulls to the other direction. In any case, it's not a great future for the big social network corporations because people can choose to have either what they offer or roughly the same without ads or tracking.
Wait Threads already support ActivityPub? I thought this was coming later?