"codebros are basement-dwelling cryptonazi freaks!" "coders are dead!" "the word 'coder' is outdated and carries a baggage of racism and sexism! what's coding anyway? Aren't we all working with computers in some way? This means we're all coders!"
2) The mainstream left is way too big to be coordinated on a mailing list or over Twitter and couldn’t agree on San Francisco if that was true. If there was never a coherent Left position on the smoldering wreckage that is Detroit why would there be on SF?
3) SF is just going to keep regulating, taxing and not building housing until there’s an event that everyone decides means SF is no longer absolutely the best place to be if you’re into startups and it’ll dissipate into the general Bay Area and the US, like Detroit and Michigan after Detroit started burning.
Seems to me that it's becoming the mainstream left, considering the demographics are younger and more migrant-y than the old-mainstream of the Clinton era. The exact rate this is happening is up for debate, of course - we'll see after the primaries.
As for why anyone would be more motivated to go after SF rather than Detroit: because SF has lots of money. Because after a certain unknown period, the budget deficits plus medicare plus social security etc will become bigger and bigger issues, and SF is just there, looking plump and delicious and terribly unjust.