The other way to look at this is apartment rental prices are merely "correcting" upwards to be less out of line with the housing prices.

A $1 million SF home has to charge $6000 in rent each month to break even, to cover cost of capital, taxes, and maintenance. Apartment rents cover utilities to varying degrees, as well.

A nice three bedroom apartment is the equivalent of a $600k or $800k condo, which "should" charge $4000+ per month. What is the going price for that rental? Across the bay area, the median is $2500. That makes sense in San Jose, but in other areas, not so much....

So is the issue that house prices are unsustainably high? Or is it the renters have gotten used to prices that lingered unsustainably low for a surprisingly long time?

IMO it is a bit of both. But we have probably under built rental units in some markets.