In Flint, this isn't a new story. Causes I would estimate as: People moved to the sub/exurbs (in Flint this is the main cause of the initial decline), got evicted and/or got imprisoned. When they got evicted and didn't get imprisoned, they moved out of the area and state, got homeless, started squatting, or got a cheaper place to rent (for instance by moving into the garage of their brother, or into the upper floor of their parents' place, etc.).

On the policy of bulldozing: Americans don't build their homes as durables in the same sense that Europeans do, many of their houses are built as cheap disposable items with a lifetime of 50 years, or so (see an example in Flint). Once upkeep of these houses is no longer being attended to, they quickly become cheaper to demolish and eventually reconstruct than to renovate.

I imagine that some people would like to live in nature, so I could imagine some more selective bulldozing. Although the reputation of cities like Flint might interfere with that cunning plan. Also, many older cities were settled on prime agricultural land, so you could try some organic farming outside of the industrial areas.