In your experience, have you ever seen or heard of a business or web site with more than 10K concurrent users who thought it was a good idea to place all those users on just one server?

connections and users are not the same thing. Accomplishing a task with fewer resources is better, so when you say " Scalability beyond 10K users/server doesn't buy you anything", you are wrong.

Imagine if you have a giant datacenter like google, and heat and power are a serious problem. Accomplishing a task with fewer servers is a serious win.

From a search I did suggest that Google have nowhere near 10K users per server. That is likely to be because the servers spend most of their time performing searches. The overhead of managing persistent connection is likely to be a small portion of what they do, like most real applications.

You seem to be implying that since, in this one use case, it's not useful, then it's not useful at all. That's a non sequitur. I already gave one concrete example of where it was useful in my past experiences.

Fair point.

sorry, that quote wasn't from you, but I think the two of you are making the same point, which I disagree with.