The only thing that stops you from having Signal group chat is you and people you talk to. When I deleted WhatsApp few years ago, I dropped a message to all my contacts informing them I'm moving to Signal and I included a link if they wanted to download it. Alternatively, I'm happy to still use iMessage/SMS or e-mail. 80% of my friends installed Signal.
My "graceful" fallback is similar to yours but Telegram --> Delta Chat (it's just email to them) --> Element (Matrix) --> Snikket (XMPP). Basically by the time they've gotten to the second step I already have a way of talking to them. I have maybe 1 or 2 people on Element and Snikket 0.
Signal’s leadership is well known for being hostile towards forks and alternative clients. It seeks to be a monopoly too. I think that’s the right call, as it maximizes user-friendliness, but if your overriding concern is monopolies then Signal is not the messaging system you should be advocating.
