This is the answer.
Matrix needs to make it easier to expire or delete messages from the server, but other than that it’s doing a lot of the stuff Signal should’ve been doing years ago. Easy to use multiple devices, easy to get messages on multiple devices, keep chat history in sync, no reliance on phone numbers for identity or single identity servers, good working federation / ability to set up private hosted groups, etc.
Much has been said about the idea of ‘signal leaving UK or EU’. Little has been said about how exactly that would happen.
AFAIK, Signal has no business presence in the UK or EU. IE, no offices, no registered corporate entities. Thus, they (arguably) have no more requirement to comply with UK’s or EU’s regulations than, say, Iran’s or China’s or any other jurisdiction where they do not do business and have no presence.
Signal’s leadership has a record of giving any regional restrictions the middle finger, so I doubt Signal would voluntarily block EU countries. So that means the EU would either pressure Google and Apple to delist Signal (easily worked around, at least on Android, and soon on Apple too as EU is trying to force sideloading) or they’d pressure ISPs to block connections to Signal (more or less impossible).
If EU tried to do that, it’d just create a giant game of whack-a-mole. And people doing real CSAM shit would just move to even more private distributed systems.