I mean, I'd say "they being in the wrong instance" is still preferable to "if the company hosting everything goes under/does something evil then everything blows up" [...]
Yes, to me as well. I never trusted Discord in the slightest and told people that there will come the day something bad happens and that they shouldn't put all of their eggs in one basket. But they still do. The average user happily accepts this risk because it's just a hypothetical, whereas they get to enjoy the conveniences immediately. This keep happening over and over again.
and you're not really removing the problems with a single provider though, as you still have the issue of "I use discord, this other person user telegram, this other instagram messages, this other whatsapp, this person for some reason only trough twitter dms" [...]
I think there's a difference between 1:1 conversations and small groups vs. communities. In the former case, it's not considered a big deal to handle multiple services to talk with friends. In the latter case, people want to interact with a whole bunch of others persistently and see what the community as a whole is up to. This is way easier when everyone is one the same platform. Forums used to fill that niche, and then came Discord and removed the annoyance of having an account for each community.
