As much as I can sympathize with those that are displace due to gentrification (And I can! as the APT’s I was living at was purchased by a developer and raised my rent so high I was forced to move. While I kept my place up and kept it nice. Virtually no one else did. )
This is often the only way that the neighborhoods can be improved.
Slum areas need to be leveled entirely and built back up. This requires quite a bit of investment by developers and they are taking on quite a bit of risk in hoping that it eventually pays off.
If the residence in a certain neighborhood could have made improvements to the neighborhood they should have.
Someone buying up all the property in a dilapidated area and developing it in a way that improves the area is a good thing in the long run.