[marcosdumay]

The one rationale I know is that there is a competitive issue, where the first browser to adopt it will have slightly slower page loads, but won't gain anything yet because no site uses it.

The slightly slower page loads is one extra round trip to the DNS server (that is supposed to be close to you). It can be solved at the DNS side, and last time I looked, about a decade ago, was indeed solved at the default configuration of most open-source servers. But the large providers don't use the default configuration of their servers.