I don’t think that ruins it for me, rather the opposite. Ashitaka is a member of a tribe far away from the places in the movie. When he gets there he is just an observer to the war between industry and nature and wants to form his own opinion. He gets sucked into it and even if he did nothing the story would have continued almost the same, bar the ending where he then has made up his mind. I’d have to watch it again and spare more than a few minutes before sleepy time to write a better response, but those are my two cents