There's a bunch of these things in /contrib which really ought to be PGXN extensions (also CUBE, earthdistance, etc.). However, one of the steps in that would be getting the mainstream platforms to package them so that users have a reasonable upgrade path, so I would not propose doing it for 10.
Part of the reason for keeping a number of extensions is that it helps test our extension infrastructure. Also they server as good pieces of example code. So I don't want to get rid of them all, or even any of them that have any degree of significant use. I think these days tsearch2 is very largely redundant, so that means there's a good reason not to keep it. But that's not true of cube, isn etc.
That's based on an assumption that PGXN shouldn't be treated as part of the community effort, which I think is a mistake. Having a robust, community run extension/package/module framework has proven to be extremely valuable for other programming environments, and IMHO we should be striving to improve in that area.
Until pgxn has a way of helping users on for example Windows (or other platforms where they don't have a pgxs system and a compiler around), it's always going to be a "second class citizen".
It's certainly part of the community efforts in many ways, but it's a significant loss of usability compared to things that are included. And from the perspective of the testing the infrastructure, you'd loose a lot of platform coverage (unless you can find a way to integrate pgxn building with the buildfarm).