The problem is everything with buildings is slow. Who can afford to replace functional buildings, and buildings remain useful for decades or more? There’s only so much you can do with infill. The only other option I can think of is to change zoning radically enough that it becomes profitable to bulldoze functional buildings. Of course that has additional environmental costs but over time should be fine

I’m personally not a fan of higher density buildings by themselves. That’s just a recipe for annoying people enough that you hope they demand better before they give up and move away. Higher density buildings needs to have some thought put into walkability, personal mobility, and transit