I mean theoretically it would be possible for people to use apps that are already cross platform, like Signal. People just care less than the inconvenience of installing an app on their phone.
Cross-platform clients, yes, but that’s only a (small) part of the way there. For example, Signal is actively hostile to other client implementations just like Apple is with iMessage, unfortunately :(
While it’s fine to criticize Signal in that instance, I hope no one discards it because of that. Things don’t have to be perfect to be better alternatives, and Signal is so far along to be a good alternative that if you would personally, idk, insist on only using Matrix or whatever and refuse to use Signal, you’d probably be contraproductive for the whole privacy and openness thing.
It’s fine to prefer something else but I think it’s positive to be fine with using Signal too.
