I get hundreds of messages a day. I don't accumulate over 100k messages annually as very few of those hundreds of messages a day are necessary to retain. What isn't automatically filtered, I look at, and either delete, archive or categorise.
I don't leave everything in the inbox for the reason that I can't find what I want, in part because the search is awful. A large part of the problem is that unlike websites where pagerank and context provided by inbound links allow a lot of intelligence, with my e-mail it is often a struggle to find the right search terms, and so I want to add context in the forms of categorisation to the messages I actually have a reason to keep.
Gmails poor folder support kept me from using it for my work account for many years, until they "surrendered" from their original hardline tagging stance and added more support to allow you to treat tags like hierarchical folders, for this reason.
I can't imagine keeping an unstructured inbox like what you're describing - I wouldn't find a thing. I always despair when I have to resort to Gmails search as it almost invariably means I'm in for frustration.