Britain tried this after WWII, building about 20 "new towns".[1] China is doing it in a big way right now. Building isn't the hard part. The usual problem is a lack of jobs.

Check out the US census population projection for the next 50 years.[2] Population is leveling off, and if immigration is restricted, it will decline. Most of the EU countries, and Japan, are already in population decline. Also see Fig. 6, "Dependency ratios". The US is headed for a situation where 75% of the population is either too old or too young to work. Retirees increase from 23% now to 43% in 2060.

We will need more retirement communities to warehouse old people until they die.

[1] https://en.wikipedia.org/wiki/New_towns_in_the_United_Kingdo... [2] https://www.census.gov/content/dam/Census/library/publicatio...