OpenStack Keystone is at /v3 already. But yeah, it really took about 3-4 years until most consuming OpenStack services updated.

For every one successful major version revamp there may be a dozen that never changed :)

Even amongst those that did do a major API revision, I suspect that you're usually left with the no-win situation of either leaving users lingering on your previous version ~forever, or enforcing a deprecation schedule and annoying a lot of people (Twitter's V1 retirement for example).