Well, that's just not remotely true. There are six supervisor seats up for election this year, as there are every even year. Ronen is running unopposed, which is a bit odd. The other five seats have at least 3 candidates each. I don't pay too much attention to Supervisor elections outside my district (which isn't up this cycle), but from glancing at websites and endorsements it looks like there's at least a few non-delusional competitors. And I recall D6 in particular was extremely competitive last time around, since it was a vacant seat after Jane Kim got term-limited out.

https://sfelections.sfgov.org/candidates

EDIT: The Board of Supervisors also has a two-term limit, so even if you think incumbents are automatic winners (which they usually are - not usually unopposed, but, like most incumbents, I don't think they usually have to sweat it too much), there's going to be a competitive open-seat race at least once a decade.

In the special election for D5, Preston won by like ~100 votes.