I know that many people that want to increase housing in SF just want to commute to their tech jobs, but that's what turned NYC from neighborhoods and communities into a hellish wasteland: too many people in a single area can't form a cohesive community. The people who already live in SF understand this, I think.
Edit: Some specifics:
The things that fall apart when density is too high. Here's an article about how density is making the subway worse than ever: https://www.nytimes.com/interactive/2017/06/28/nyregion/subw...
Schools overcrowded: https://www.dnainfo.com/new-york/20170921/riverdale/overcrow...
Traffic congestion: https://ny.curbed.com/2018/1/18/16903152/nyc-traffic-congest...
Trash infrastructure overwhelmed: https://citylimits.org/2015/04/20/new-report-nyc-trash-is-wo...