I mostly get the impression people leave because they don't want to deal with the school lottery system. As for retirees, they tend to have incredible housing wealth built up - and old people being replaced by young people over time, even if bad, is much better than the alternative.
> A few years ago I read that the average tenure of a business on Valencia was less than 18 months, which is nothing short of a death sentence for a neighborhood like the Mission.
If you want to have businesses in the Mission, they need cheaper startup costs. That doesn't happen when SF is designed to make new businesses take 2-3 years to open so the planning commission can determine they're sufficiently quaint and precious, and all their neighbors and competitors can sue them looking for bribes.