Matrix bridges solve a lot of this problem, though... aren't really reducing complexity at all ends of the system. It does radically reduce end user app complexity, though.

I've been hosting a Matrix homeserver for... oh, 4-5 years, now, and I have bridges installed for my use and a few other people who use it that bridge Signal, Google Chat, Facebook Messenger, and maybe one or two other services into Matrix - so I almost never have to bother with the other clients, I just use a Matrix client everywhere. There are the occasional quirks you have to deal with, most of which are solved by upgrading your bridge (and the new bridges are a lot easier to deal with than the older ones).

As people decide to go Matrix-native, I can talk to them that way as well.

That said, as far as non-Matrix options go, Signal seems to be a fairly common one and easy enough to get people to switch to.