The exceptions are web-apps that require heavy backend computation (javascript/node sucks at that), but even then you could use node + a work queue that's handled by a more appropriate language.
Nodejs has arguably the best websocket implementation of any language and has mature drivers for pretty much every database backend. It has various templating frameworks etc.
A BB just needs a database at the backend and a way to get that data out to a browser. In that regard, nodejs is more suited than many other languages.
I don't really get your comment since it seems to imply that writing it in nodejs comes at some cost and you need to justify that cost by saying you expect a lot of traffic. I've seen this attitude before when someone writes something in assembly.. "Oh, you're hand-optimizing your program's assembly? Unless you really need that performance I wouldn't think it's worth it to use assembly". The thing is, nodejs doesn't have all that many problems. Sure, there's the massive number of warts of javascript, but people have learned to write readable and less-buggy code in the javascript ecosystem at this point (promises, jslint, etc).
So to repeat my answer to your question: no it doesn't have to be nodejs to be "modern web", but I do think nodejs is appropriate for this use case.