He's not leaving Bitcoin as a developer. "Maintainer" is a more specific role in open source generally, and in Bitcoin specifically - you have special rights on the code repository, to push/merge new code to the 'main'/'master' branch and similar things.

We generally want at least "a few" maintainers, rather than just one, to avoid central points of failure. But we don't really want people to have that maintainer role, if they're not using it, since it's a security sensitive role.