I've seen the effects of too few cars at SFO recently -

I've been given a Sky/Solstice because they didn't have what I booked (OK by me as it happened, but could easily have been a problem)

There have been a couple of times when they were booked out and no availability for a given date 1 week out.

However, I have had quite a few cars with <1k miles on over the last few weeks.