Imagine if we grew up in a world like the one you posit, where humanity advances only slowly because it's held back by the ideals held by people 400 years old.
You propose that the best solution to this is kill everyone over 80.
Or, imagine that you didn't like the dominant views among the 50-60 year olds in power when you were in your twenties, and you offered up the idea that we should just withold medical treatment from people over 30 so that we could increase the speed of social progress.
You propose that the best solution to this is kill everyone over 80.
That would make you a psychopath.