They can already do that. Using an alternative does not cause someone to lose access to FB.
> There's absolutely no technical reason for everyone to have a separate Facebook/Instagram/Twitter account with dozens of other IM services instead of picking one and communicating with others regardless of what they've picked
This is already a thing.
> Using an alternative does not cause someone to lose access to FB.
Deleting Facebook causes you to lose access to your Facebook connections. Why can't you just use your Twitter account to communicate with someone on Facebook? There's zero technical reason for this not to be possible.
> This is already a thing. https://www.pidgin.im/plugins/?publisher=all&query=&type=
It's not. It's a third party workaround that Facebook could kill at any point if it gave a shit. Do I really need to explain to you how that's different than first party support for something so basic? Just like with email, you should be able to enter a username and a IM provider and just talk to them as if you're using the same platform. It's really not rocket science.
Fediverse (Mastodon and compatible providers) already does that without any hiccups. It's possible, we know it works for decades (since we all use email), and it should be a requirement for every social network / IM service in existence.
That future is only one regulation away. The most common reason for staying on Facebook ("all my friends are there") disappears.