I doubt Signal would integrate. It would be a cold day in hell before Meta implements Signal's E2EE protocol. And Signal removed SMS support, so I think unless they had full E2EE they'd simply pass on integrating.
I’ve got different apps and talk to different people on each. It’s not like I’m splitting one conversation across two apps. So when I want to talk to someone (or some group) I use the app where we’ve been having our conversation. Switching cost is high and all the major apps have extensive feature parity. In the rare instance where myself and/or anyone in a conversation has a need for a feature that’s only offered in another app, then we’ll move the conversation there. But we’re not moving all our other conversations and “I want to consolidate” isn’t sufficient justification to ask someone to move theirs.
Maybe this is because we already consolidated our separate communication channels into one mobile device, which serves as a consistent form factor for delivering vendor-agnostic chat experiences. Just like Trillian provided one interface to receive notifications and initiate messages, so too does my phone. I get notifications from WhatsApp and Signal on one screen, that I can control with one focus mode, and navigating to one or the other requires a tap or two on my screen. The problems of maintaining separate AIM, MSN, and Skype accounts just don’t exist in the same way they did back when Trillian had obvious appeal.
Ironically the fragmentation has actually reduced the power of (non-interoperable) SMS protocols, since chat apps are so common that any pair of people can chat without depending on it. But if anything should be interoperable it’s the OS-level chat APIs like iMessage, which only exist because of historical SMS dominance and now find themselves in the weird spot of being just another app-level messenger.