But with open source, it’s a lot harder to move the community to do something that users have never imagined they want.

I'm pretty sure he was referring to the developers, i.e. the people that would actually have to implement those changes in Core or those who would have to upgrade their plugins and themes. That's why backward-compatibility is so important.

So, from this point of view, Capital P fails as a counter-example, since it was trivial to implement and had no back-compat implications.

The only thing you have to convince end-users to do is upgrade to the latest version.