My parents are not boomers (they're Gen X), but kept their marginal mortgage because the capital they would have used to fully pay off the mortgage could be better deployed in a mixture of investments.

A lot of households have done something similar.

Additionally, not all households bought their first house in their 20s - plenty of households did so in their 30s and 40s.

> They could buy a house when you could easily get one for <10 years of salary almost anywhere

And during that era, you had double digit interest rates [0]

[0] - https://www.statista.com/statistics/1338105/volcker-shock-in...