I expect rents to go up considerably, given that people are avoiding buying houses, population is increasing (expected to be 500M by mid-century), and inflation is high (I don't see any way out of the US government's debt mess other than inflation to reduce its real value).

So in 10, 20, 30 years a renter will be paying considerably more while a buyer will be paying a rate locked in long ago.

Imagine the relative pittance you'd be paying now on a loan locked at 1988 housing prices - and that's after 2 decades of inflation rates that were lower than those of the next 2 decades will probably be.

I bought my house in 2001, and for me to rent an equivalently sized house now is already $600 to $1000 more expensive per month than my mortgage. It was a bit worse a couple of years ago, but still; people who forget about rent increases are in for a big surprise if they could have gotten a house for a reasonable price.

That doesn't seem to be true. Doesn't "irrational exuberance" talk about that? Something can be the case for 20 years, but then change.

Sure, but the point is that "constant rent for 30 years" is a pretty lousy assumption.