Turns out the logic behind buying a home based on the rent-to-sale ratio is:

Your fixed rent expense for a year is roughly equal to a year’s mortgage payment. No matter what, you have to spend this money—so you might as well buy a house.
If the rent-to-sale ratio is too high, then renting is basically “getting a free ride” at the landlord’s expense—especially in the early stage of a new home, when the decline rate of the property’s losses is extremely high. If you stay for one more year, the landlord is the one taking a massive hit for an entire year.
Then you can build a quantitative model of renting and the landlord’s losses, and finally come up with a hedging plan.
I call this the Soros renting hedging method.