San Francisco uses ranked choice voting or IRV, not a two-round runoff. I should've used a different term to avoid confusion. Here's what I mean by a two-round system https://en.wikipedia.org/wiki/Two-round_system

I don't think that the voting system has made SF 'essentially a one-party system', as tons & tons of cities across the US just use normal FPTP and are still very Democratic. Just as tons of rural states just use normal FPTP and are still very Republican. This is what I mean by, voting systems are overrated and just don't have the large effects that enthusiasts want them to have. It's a seductive argument for left-brained engineering types who are systems thinkers, but it's just not empirically true