30+ years isn’t going mainstream as long as QM is in place, since it requires 30 year max, fixed-rate loans.

This being said, if a 100-year fixed product existed, and principal+interest were lower than a 30-year fixed, then it would make sense for some people. Especially if it were assumable.

Say you’re buying investment property in a major city (NY or SF) where cap rates are low, intend to collect the income and NEVER sell, but rather pass it to your heirs. A 100-year fixed loan would make a hell of a lot of sense in that case.