Signal is still an improvement over other non-federated messengers in that it's open-source, so you actually can try to improve the situation, although it's notoriously difficult. As an example of more platform support: https://github.com/signalapp/ringrtc/pull/12

signal-cli is an example of a 3rd party client which is tolerated for now: https://github.com/AsamK/signal-cli

The main problem right now is that they don't have enough developers to take care of everything, but it's not specific to centralized services (no developer == no code). If you care about it, you can develop your own client using their library (à la signal-cli).

Regarding your last paragraph: I could probably list 20 features I'd like to see in Signal. That doesn't mean I want somebody implementing them with no guarantee about how securely they are implemented. One of the main goals of Signal is to provide guarantees against dragnet surveillance, and that constraint takes precedence.