The history of every TCP server in Unix goes something like this history of inetd:
1. inetd would just accept() immediately and fork a copy of your service (say, ftpd) every time a connection came in.
2. That went bad when you got too many connections, so they changed it to do a maximum number of forks per second or per minute (aka a "QPS limit").
3. That went bad if you had long-lived connections, so people wrote things like xinetd (or djb's tcpserver) that limit the total number of live connections and refuse to accept() until one of them finishes. (In non-fork-based threaded services, this limit is usually implemented with a thread pool.)
Seems Go's HTTP server is still on step 1. I would make a snarky comment about what year that's from, but I actually don't know, because inetd already did step 2 when I started using Unix in 1993.
Step 3 (xinetd) is way more recent, I think maybe 1996 or so. So Go's way of doing it isn't that obsolete really. Only 17 years.