If I were able to devote more time to just testing Drupal for the sake
of testing it and with no further purpose, I could and would use
Postgres more of the time. But even then -- if my goal were to test
the new menu code, I'd want to eliminate any other possible error
causing sources, such as using Postgres.

So unless we actually push the issue of supporting multiple databases
as a community, the vast majority of developers will never make the
effort. And with each passing release spent in that mode, the harder
it will become to reverse it. We will end up supporting only one
database, MySQL, de facto.