Does "modern web" mean it has to be Node based? Unless you are expecting a hell of a lot of traffic, I wouldn't have thought a BB was a particularly suitable use case for node (correct me if I am wrong).
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.
Maybe I just too traditional, but there are far more mature back end frameworks available, without the warts of JS. For me I prefer to see a need or justification of using the latest software trends, not just because "its modern" or "that's how we do things nowadays" (a genuine response to a project I am involved in when I asked the case for using Node).
Node.js obviously has some use cases where it will shine, but I wouldn't have thought a BB was one of these places, as its quite a traditional kind of app.
Not to mention this thing is a pure, pre-ejected abortion of design. https://community.nodebb.org/ I'm not sure who these web designers think they're fooling with this crap. The "modern web" looks full of hipster goons who would have also cooked up the ribbon interface. I'll stick to my minimalism, black text on white backgrounds, usable forums, and not reinventing the blink tag.
I think with nodeBB we can safely say, NEXT!
I feel like I just traveled to the past.
I've been following these guys since the start and I think they have done a pretty good and refreshing job. They share some features with discourse and vanilla forums because they are the 3 competitors fighting for the modern BB software cup...
BB has been quite stagnated for decades, it is actually nice to see some "innovation" from all these projects.
(You are aware that all LAMP stuff is also FOSS)
We really need to explore new ways of interaction, and blending old fashioned boards with the new "real-time web" is a nice start. There's nothing bad in trying new things. Discourse, with rails, is another good take...
PS: I use a lot of LNMP for 70% of my freelancing projects, but for a forum that needs real-time capabilities Node is a convenient option.
I hope this comment gets picked up by @shit_hn_says.