I think that this is a valid point.
We want to be able to decentralize, and allow people to work on their own volition. In a lot of cases having it as their own project is worth while.
One of the reasons why joinmastodon.org is currently pointing at instances.mastodon.xyz is because we (other people than thekinrar) have not been able to make a instance picker go live on our own.
If we find that instances.mastodon.xyz does not fill the needs we (mastodon project/joinmastodon) want filled, we can fork, as we have been allowed to reuse the code.
But right now, it is very good that it's someone else's project. Mainly because we wouldn't want to be a bottleneck with development, more than we already are.