"Gentrification" is not wrong.
A neighborhood is either desirable or expensive or undesirable and inexpensive. It can move in either direction for a variety of reasons.
Maybe a community loses its industry and there are no more jobs, so prices go down, but it's still a bad place to live because there are no jobs and no future. Maybe hot new industries come to town and the opposite happens.
Maybe crime goes up, and people who can afford to leave move away. Maybe crime goes down, and people who used to avoid the neighborhood start moving in.
The list goes on and on.
Suppose we have a low-cost area that is in reasonable commuting distance from high-paying jobs. Why is it low-cost if it's so well-located? The obvious answers include crime and dilapidation. There are two ways things can go from here: The conditions improve and the prices go up, or the conditions stay lousy and the prices stay low. The "anti-gentrification" crowd tells you that price increases are immoral, but are they really ok with crime-ridden slums? Of course not. They promote magical thinking - the idea that we can get rid of crime and fix up the buildings without increasing the prices, even though the only reason that the prices were low was the conditions!
All of this without even mentioning that everyone who OWNs in a "gentrifying" neighborhood wins the appreciation lottery.
Shorter commutes are more green. It's better for society as a whole if the people working the high-paying jobs live near them. People doing little to no work don't have any particular reason to prefer one area to another, nor should we pander to them. The idea that you are more entitled to live in a particular area if you "started out there" has terrible implications and quickly becomes oppressive.
Finally, of course, the elephant in the room is identity politics. Most ant-gentrification rhetoric usually stems from toxic identity politics. People who get very angry when Identity Group A is displaced by Identity Group B don't care one whit when the opposite happens somewhere else. At best, such people are consistent in supporting the displacement of "richer" people by "poorer" people, or at least those they imagine to be so, while opposing the displacement of "poorer" by "richer." But adding emotional rhetoric doesn't change the facts above.
There is no good reason to keep crime in place, to keep dilapidation un-remedied, or to maintain filthy hovels when better alternatives are available.