Firstly, it's not the goal of the PostgreSQL project to overtake MySQL or other databases, but to serve the existing/new users. This also means we're investing the development effort in a the highest benefit / effort ratio. Even if switching from process-based to thread-based model improved the per-connection overhead, the amount of work needed is so huge the benefit / effort ratio is so utterly awful no one is going to do it. There are always better ways to invest the time / effort. Especially when there are practical solution / workarounds like connection pools.

Secondly, every architecture has pros/cons, and switching from processes to threads might help in this respect but there are other consequences where the process model is superior (some of which were already mentioned). Focusing on just this particular bit while ignoring the other trade-offs is rather misleading.

And no, the arguments did not really disappear. To some extent this is about the programming model (locking etc.), and that did not really change over time. Also, PostgreSQL supports platforms, some of which may not have particularly great threading support.

I'm not claiming there are no workloads / systems that actually need that many long-lived connections without a connection pool. In my experience it's usually "We don't want to change the app, you have to change the DB!" but fine - then maybe PostgreSQL is not the right match for that application.