I don't think people realize how much the mortgage tax deduction has neutered itself and become a burden on the tax system. Yes, it makes houses more affordable, so what this means is that houses have become much more expensive than they would have been otherwise. The deduction is priced into sale prices. The mortgage deduction feeds housing price inflation without making houses easier to buy or contributing to faster growth.
It's like heroin and we need to wean ourselves off of it, slowly enough to prevent a total economic meltdown, but deliberately and completely.
You stated it perfectly HostileTerritory.
A subsidy always makes things more expensive if ALL the cost factors are included. However, we know the government figures (correctly) that most of us never look that deep into the entire picture.