Another is that some of the current housing supply will be eliminated because it is dilapidated. So even if the birth rate were constant, there will always be housing that are destroyed. I bought my first house 3 years ago, and my wife and I decided to only consider brand new houses because I am terrible with home improvement and fixing things up. Even if a house from 1995 is in decent shape, many people would just rather have a new house.

Second, even if the birth rate is constant, you can have net new immigration. This was discussed below, but through legal and illegal immigration you will still need housing for new families coming to the US.

Third, birth rate is different than the number of house holds. If people are getting married less and/or divorced more, then there will still need to be more housing. Since marriage rates are declining single individuals will either have to live with their parents, (plausible in some cases, but not always), or buy their own house; whether they have kids or not.

4th, people living longer. Even if birth rates are lower, if the average life expectancy continues to increase we will need some housing for the seniors. Of course some of them will live in assisted living centers but now quality of life standards are going up and you can be independent well into your 80's. This is a big reason the housing market in Tampa is exploding, you have rich, retired seniors moving down from the Northeast. 20 years ago they may have gone to an old folks home but now they want a condo or stand alone house.

5th. Prices are only relevant to specific areas/neighborhoods. If people continue to leave colder areas and expensive cities like NY, San Fran, Boston, Philly, etc., then the prices in Utah, Colorado, Arizona, Florida, and Texas can still continue to skyrocket; at least until we've reached equilibrium between the popularity of the two areas. But even if Utah, Colorado, Texas, etc., had real estate prices that were on par with NY/California, it still would be a desirable place to live because of lower state and local taxes, pro-business rules, etc., In theory if all of the people left NY/CA and they voted in changes to the new states making them less "friendly" like raising taxes, then people would go back to NY/California... maybe.