I've been replacing my 11 speed chains early too. And, so far am getting quite a pile of chains without destroying the cassette. I still need to decide what to do with the half worn chains. Perhaps I'll choose a bike that I will want to use slightly stretched chains on.

So, the life of a cassette? I've blown out cassettes with a single over-stretched chain. And, kept cassettes in good working order with very careful attention to stretching.