The point is that these are parts of cities that have *already* been abandoned. The population of Detroit has dropped by 50% since it's peak in the late 50s. There are whole neighborhoods that are effectively empty. Do some quick googling and you can find lots of images of whole blocks where weeds and trees are taking over already -- the pavement broken up by plants, trees starting to grow out of houses, etc. Same thing in nearby Flint. It's that TV show "After Humans" in real life in those places.

The only question is do you let these neighborhoods go back to nature at their own rate -- with lots of opportunities for mischief as they lay rotting. Or just acknowledge that half the population of your city ALREADY left, left many years ago, and you might as well turn the land into something with fewer crime and fire hazards.

Really not much different than mining "ghost towns" that were abandoned after the ore played out except in scale. Economic conditions changed, jobs shifted, and people on their own made decisions to leave -- that some cities will sometimes fall behind seems perfectly compatible with vibrant capitalism and liberty.

Now if someone were proposing that we flatten still-occupied sections of still-growing cities just for urban planning purposes, that'd be something very different, but that's not what's being talked about.