Dino has an intentionally simplistic design, but it doesn’t look “dated” at all.
That's highly subjective, but I've shown some Gtk3 apps to people at work and the most expressive first reaction I got was "ew". Dino and others getting that exact reaction wouldn't be surprising.
it just doesn’t have a major centralized provider of them that in exchange siphons up all your personal data like Element & Beeper does. But you can easily self-host the available bridges for XMPP
And this is another reason why it isn't prominent. "Grandma, all you need to do is host an XMPP server. It's incredibly easy".