what happens in your decentralized system if one of your friends has their account registered on a naughty naughty server that just got blacklisted by the righteous admins of your instance and now you can't talk together anymore? does that person need to make a completely new account just to send some messages to you? does a migration scenario exist? and if it exists, does it warn your contacts and transfer everything seemlessly?

Last time I checked Matrix was researching about the topic of multiple homeservers. So you could just use a local homeserver on your device and a remote homesesrver with a fancy name simultaniously. This would also enable users to communicate P2P. But I don't know how advanced this system is yet.