Gentrification is a "dysphemism" (opposite of euphemism). It's basically a buzzword made up to give a negative connotation to something that isn't inherently negative.

People move around. Some cities grow, others shrink. New jobs get created, old jobs disappear. Some housing gets more expensive, some housing gets cheaper. Some cities get wealthier, others get poorer.

We'd still be living in caves if our goal was to forever keep all cities and towns the exact same way they've always been. Which is basically the goal when people criticize "gentrification".