Suggesting that we build more housing as a solution to a housing shortage is silly? I think suggesting that we don't build as a solution to the housing shortage sounds a lot sillier.
Smaller units aren't necessarily for families, they are usually for young people who are starting their careers and want to minimize costs. Have you not heard of the concept of a starter home?
Allowing property owners to exercise their property rights tend to benefit the property owner. A middle class family can add a unit and rent it out to add a revenue stream, how would something like that not be helpful? Doesn't even have to be rented out as a residential unit, in a perfect world we would see accessory commercial units in addition to accessory dwelling units.