Economics

Aside from all the other ways this proposal is stupid, a 30-year mortgage at least makes some sense compared to the timescale of a typical working person--at least historically. You get your career going in your early-mid 20s. By 30ish you are stable enough in your life and career to make the investment into a house. You make those payments dutifully for 30 years, and then by typical retirement age you own your home free and clear, and your largest individual living expense is gone.

With a 50-year mortgage, you are guaranteed to have that debt hanging around your neck well into retirement, or worse.

And yes, I'm aware of all the hand-waving and outdated assumptions in that first paragraph, don't @ me. Broadly speaking, the point is that the long-term implications of 50-year mortgages becoming common are pretty terrifying, since it's virtually impossible to expect to pay one of these off during period you are gainfully employed.

You pay for 30 or 35 years, then default in retirement (or die). The banks earn exactly as much in interest from you during that time period, compared with a 30-year loan, plus they get your house at the end.