@HistoPol @jamie @oliphant @snarfed.org @luca @PCOWandre @chronohart @snarfed missed this earlier, but the way out isn't usually account migration, that's a very specific and non-standard function that Mastodon implemented (Mastodon has a bad habit of only half-implementing ActivityPub and then rolling out it's own features and forcing everyone else to comply with their non-standard nonsense... they're kinda the internet explorer of the fediverse)
The way out is because you can leave without severing connections. I'm not saying you won't have to re-add people... but that you still have the option to re-add people. If someone deletes their Twitter account... they lose access to everyone that's only on Twitter. If Bluesky is bridged (and no ifs ands or buts, open bridges will exist despite people's complaints) and they delete their Bluesky account... they'd still have access to those same people.
The whole reason Facebook sticks around and maintains enormous power is because so many people don't have the option to leave without making themselves second class citizens in their communities. (Literally the only reason I have a facebook account... if I could access them over fedi, I'd delete my account in a heartbeat)