If your setup works fine and doesn't need any maintenance, there's really no reason to switch to something else. But once you upgrade to a newer version of Sphinx and it crashes, I personally feel more comfortable knowing I can report the issue on GitHub and expect it to be fixed eventually. Unfortunately, that's not how it works with Sphinx.

Speaking of features, both Sphinx (as a closed-source project) and Manticore (as an open-source project) have added some nice improvements during last years. But again, if you're happy with the 20-year-old version, there's probably nothing to worry about.