These experiences aren't undone. They are stopped. There is a difference. Something happy that happens, and then is over, still counts as a happy thing.

You destroy valuable lives. You also create valuable lives. If creating things has as much value as maintaining them does, then the act of creative destruction is morally neutral. Since the only reasons that I can think of why maintaining lives might matter are also reasons that the existence of life is a good thing, I think that maintenance and creation are morally equal.