Let's assume communities are rated on a scale of 1 to 10. A "gentrified community" goes from being a 3 to being an 8. Renters are forced to move because they can't afford 8/10-rated-community rents, while existing owners profit handsomely. On this we all agree.

Where do the new residents of this now-8/10 community come from? Probably a place that was less than 8/10 - maybe it was 7. So now there's less demand for all the 7s and their rents decrease, allowing residents living in 6s to move there. And so on.

Assuming housing construction in the region has kept up with the population, even the renters who were forced out of the previously 3/10 community will likely find new housing in a 4/10 neighborhood at the same price. Their relationships from the old place were probably disrupted by the move (bad) but they also got better housing for the same money (good).

The key in this is housing construction must be allowed to increase with population.