The key question will be how to cover the cost of maintaining existing trees and planting new trees. The SF Dept. of Public Works and FUF can't keep up as is. The best way to finance a real greening of SF is probably going to be a city-wide parcel tax. That would take the burden of existing trees' maintenance off of property owners, and it would enable the City to actively plant new trees in sidewalks.
Like all tax measure in SF, anything like this will require a coalition of all varieties of progressives. Are you in?