In the rental market a big driver of the landlord decision cycle is needing, at a minimum, to have a reputable tenant and to cover the cost of carry of the asset (mortgage, etc). In many cases if their cost of carry is met (and that's often a low bar....many landlords in both NYC and SF own their apartments outright, or are paying tiny monthlies on a refinanced mortgage that was originated in the 90's), then the priority is getting a reputable tenant who won't destroy the place or create drama.
If the above paragraph is confusing, basically what I am trying to say is that a landlord often prefers, for example, $3000/month from a tenant who they think is 98% likely to be an "easy tenant" to $3500/month from one who they think is 80% likely to be the same. In other words, there's a market premium on reliable tenants, especially in places like SF that have aggressive tenant-protection laws which make landlords even more antsy.
As layoffs start happening (as they did in the nine months before we signed our lease back in '09) landlords of reliably-paying, drama-free tenants start getting antsy about keeping their current tenant or finding a suitable replacement.
As such, what drives prices in markets like NY/SF isn't just supply-demand equilibrium; there's also a significant behavioral economics angle aspect to it as well, as landlords are willing to pay a premium for peace of mind.
Therefore, I would expect a significant drop in SF rents if layoffs start coming.