If I was to decide whether delete every altcoin or leave the situation as it is, I would keep all the altcoins. Because they are the history or knowledge. They demonstrate how people can be naive and believe in lies, how people invest in things that will never make them rich or will opposite - make them rich easily. Altcoins show how different can be cryptocurrency world.
Moreover I think every alt plays huge role in overall developing crypto world of the future. I mean most of alts are just bad experiments and we can only reach great results by testing out what works and whats not
If we delete all alts - we'll lose track of our past experience so that would be a huge leap backwards in terms of crypto industry growth