OK just a couple of points here. I'm not gonna be brief because I care about all this. Sorry.
De-federating from Threads is not the same thing as de-federating from the BBC, it's another issue entirely. Those who did it explained their choice with the fear that Meta could somehow "embrace, extend, and extinguish" Mastodon, plus with the fear of data collection etc etc. Now I'm not saying they are right (I don't even know where I stand on this), but if those are their fears, we're talking about the destruction of Mastodon itself. Which is not even comparable to what the BBC's instance could do.
About the trasphobia itself: what the BBC did or did not do is besides the point: the BBC is too relevant to just block it willy-nilly, and also very reductive. If you block it, you throw away the baby with the bath water.
I would also dispute the idea that the BBC is "largely conservative", but even that's beside the point. Let's pretend that it is: so what? Being conservative is not a crime and not all conservatives are Trump. I'm not conservative by any means, but I still want to see and hear what conservatives think. As a left-leaning dude, I WANT to know what they are up to.
My fear is that we're weaponizing the Fediverse to create communities which are completely sheltered by the actual world. For all its flaws, Twitter was great in that it showed you a bit of everything. I don't want to see the Fediverse become a series of spaces where people only agree with each other and don't even want to engage in a discussion with someone they don't agree with. What we're both doing right now (disagreeing and debating) is so much more valuable that people think.
Lastly: being on it since 2017, I know full well how the fediverse works. And no, migrating from one Mastodon instance to another is not easy by any means. This article gained some traction recently and it explains why. But even this is besides the point. First, because ideally, you should not have to migrate to another instance. It's possible, but is sucks. Second, because I'm talking about some cultural aspects of the Fediverse, and bringing the discussion to a technical level is a moot point.
My question and my whole point is this: is there a risk that the Fediverse is becoming an instrument to isolate ourselves from everything we don't agree with? I.e. an instrrument of isolation instead of an instrument of federation?
You had to name it like that, didn't you?