Playing devil’s advocate here, there are some talks that are constantly applying to new groups of people. The developers that learned from your talk last year are better now and don’t need it, but new ones do. Just because there is some overlap from one year to the next at a conference doesn’t mean it’s 100% (what is that percent, do we know it?), and your talk might appeal to all the new people.
Having said that, there’s clearly a balance to be had and there might be some good guidelines that could be suggested to help event organizers find that balance.