Guess it really depends on how many concurrent users you expect to have...

Sounds like 30 GiB RAM for 1 million concurrent users is not unreasonable:

https://stackoverflow.com/a/17453704

Also your load balancer stack needs to support it, which is becoming less of a problem these days but there's still some stragglers. You could also end up with an imbalance where some servers have a lot of connections and others have few.

Yes that's some genuine concern. Though on B2B and B2C SaaS where you got paying users, I think each of them deserve a websocket, and it could* alleviate some headaches for developers.

* Anecdotal of course, at least for us.