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.
Im a fan of manticore and promote it when I can, but this doesn't seem like an approoriate response. It appears generous on the surface, but has various unnecessary and subtle digs in it - all while it would be easy to show how manticore is a more appropriate choice.
They're surely not using a 20-year-old version of Sphinx - it appears that the latest open source version on Github is 8 years old. They have a closed source version which appears to be maintained, though has infrequent releases.
Why not just say these things, which make manticore self-evidently a better choice?
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.