[ryandrake]

But there is so much inefficiently-used land and so many cities that you don't have to deliberately go into an existing town and change its nature head to toe.

There are plenty of already-urban areas that have poorly-used lots that could benefit from building more urban housing. Why not start there? Keep like housing styles together: If you're the one single family house in the middle of a city surrounded by dozens of apartments, then yea, that lot should probably be re-developed into an apartment. The single family home is out of place and the neighborhood is already set up for dense living.

But if you're in a small town that's all single family houses, it doesn't make sense to re-develop a random sampling of them into apartment buildings. 1. They'd look out of place and 2. These small towns don't have the infrastructure to suddenly 3X-10X their populations. They'd need more schools, transportation, electric capacity, water/sewer capacity, trash collection, retail, industry, everything.