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.