You stated that "progressive supervisors are focused on public investment in affordable housing and taxing the rich to fund it." This is not what Prop I does.

Instead, the >$10M real estate tax would mostly be paid by developers, not homeowners, because most property sales in San Francisco that are >$10M are apartment buildings rather than houses owned by rich people. Not only that, but it would be a tax paid twice by developers, because they have to buy the land and then sell the finished development.

The reason YIMBYs opposed that proposition is for exactly this reason. It wouldn't be a tax on rich people, but yet another obstacle to the construction of housing, both affordable and market-rate.