You can't do 5+1 on the current total land area occupied by low-density because you'd end up with far more housing than 330 million or so people need. You add that level of density to the core of small towns, then you have enough for everyone. That leaves a long tail of abandoned infrastructure in the areas that you're describing (rural/exurbs not picked for 5+1 upzoning).

The suburbs take a huge amount of land. If everyone lived in NYC-level of density, we could all fit in New Jersey. If we all lived in middle-missing housing level density, that amount of land is still far less than the total area of all the suburbs.