It's not arbitrary, I'm just sampling the most populous because that's typically where the increases have allegedly been the most dramatic. The intention behind choosing only the top 10 is to be as minimally charitable to my own point as possible.
In the top 20 most populous MSAs, only 7 experienced a substantial multiplier increase.
In the top 50, it's even less.
The further down the list you go, the less likely there has been a dramatic increase in home-price-to-income ratio.
There's a whole collection of "Fine, American cities" (https://twitter.com/jmhorp/status/1221828340656431105) where multipliers have either not increased, or decreased.
Yet again why the arbitrary limitation? Your own source shows about a dozen areas going from the 5-8 band to the over-8 band since 1990. More than two dozen going from the 4-5 band or lower to the 5-8 band. When each multiple represents most of $100K. When the entire graph should ideally remain about the same color overall but is clearly getting worse. Buy hey, let's look only at the top 10 out of 392 and ignore the fifth of the population that's not in an MSA. That's totally the way to "systematically isolate root causes" ... so long as the root causes are the ones we find convenient. Otherwise not.