Right now the big mantra is that part of the reason that houses are unaffordable is because the mortgage rate is "too high" at 6%. (still 1/2 the rate that was being charged in the mid 1980s.) The idea of the 50 year mortgage is that it "buys down" the payment by approximately a couple of hundred dollars a month on a $2000/mo mortgage.

It's basically a hack to lower house payments without lowering the prime interest rate (which would throw nitroglycerin on inflation and tank the dollar.)

It's just another financial game.