The company (Signal Messenger LLC) is fully owned by Signal Foundation, a 501©3 non profit organization.

OpenAI is also non-profit. Not really an argument.

Probably around 80-90% of Matrix users are on the matrix.org homeserver, so it’s absolutely not as decentralized and resilient as you think it is.

Well, the goal is that moving to your own server, will not mean that you will loose access to all your contacts. Which makes moving instances much simpler. If Matrix gets a hostile take-over, your don’t really need to reach a critical mass for an alternative server.