I disagree. If that was the case, pgBouncer wouldn't need to exist.

The problem of resource usage for many connections is real.

It's about queueing work, not running all these queries at the same time. You can run pgbouncer or you can have a pool on your backend. Having more connections won't make it go faster, so that really seems like a low-priority thing for postgres to me. Even if you integrated pooling into postgres the overhead of auth would be still taking time for small queries anyway.