I love Signal, but at the end of the day they still operate a centralized service with all the drawbacks that entails. It only takes a change of leadership to kick of progressive enshitification, just look at what happened to WhatsApp. Being run by a non-profit should help, but the chance always exists with centralized control. Also their multi-device support is still not great, no official support for Android tablets for example. And idk why not, because Molly (Signal fork) recently added that without too much difficulty afaik.
Session looks really interesting imo, kinda like a decentralized and multi-device version of Signal.
Not everyone is a FOSS maximalist.
Imo multi-billion dollar companies are the big problem, because they often have monopolies and will use them to push crap down our throats. Much smaller companies can still offer good software and support, and they are sometimes the only viable option. I'd prefer it if everything was open-source of course, but that's just not how the world works right now.