I'm like you (in fact I delete everything after looking at it, effectively only storing what's quoted in my sent box) but yes I think most people do just let emails pile up, from what I've seen whenever I catch a glimpse of anyone else's inbox. They get emails that they don't want and never open, but also don't bother to delete them or unsubscribe until they hit a storage limit.

I do love the elegance of searching with the "unsubscribe" keyword, but it doesn't solve the problem of the inbox filling up with junk again. I wonder how dangerous it would be to put in place a filter rule that deletes anything received with that keyword. I think the next most common phrase is "manage your email preferences" or "communication preferences." I'm curious to search my wife's Gmail inbox with these terms to see the precision and recall for myself.