I'm not sure what parts of Detroit you're talking about, I can't think of a single neighborhood that is struggling with that problem, blight and abandonement? Sure.

Blight is a direct result of people in the area being to poor to afford to maintain infrastructure.

As to driving prices down, some houses in good condition are selling for ~25-50k which would be worth more basically anywhere else in the US.

That's not because a bunch of poor people moved into the neighborhoods. That's because the middle class moved away or lost their jobs.

It’s not just a question of the middle class moving away, people die which means you need more people moving in. As soon as the mix of people moving in starts to include less affluent people it eventually gets dominated by them simply because their are more people as you move down the income curve.

The transition within a city is rarely about some specific employer shutting down. The more general case is homes age and different areas become attractive over time. You can find areas where nearly identical homes where build across a wide area, yet one community falls into disrepair while another becomes desirable simply based on school zoning.

Honestly, Detroit had nowhere to go but up. The city was so poor that even a decade ago downtown was a hot mess where there was virtually nobody there after 6 PM.

I think you have it exactly backwards the cost goes down due to the area being a less desirable location allowing the poor to afford the houses there. If you analyzed the situation better it is extremely likely that there is an underlying cause that isn't poor people moving in. The economic situation in Detroit has been shit and getting shittier for years now leading to the people who are more capable and therefore able to command a higher salary to tend to move elsewhere.

A house that a doctor and a lawyer are competing to buy is going to be go for more than the one an auto mechanic and a fireman are competing for.

It is a little bit of both, at least when it comes to maintaining a local public school’s rating. People will oppose lower income housing to prevent the children of lower income people from attending the same schools, which might cause the school’s rating to go down. Which would then be downward pressure on the price of the land.

Every real estate website has school ratings for each house listing because it is such a sought after amenity.