I think your use case is not universal. I regularly reach 80% - 90% and so do my peers. We have to regularly use findbigmail and smart-personally-defined filters to delete big and unnecessary mails.

Gmail is used as a collaboration as well as backup tool by at least a few of us. What it means is that we often tend to many attachments exceeding 10MB - and multiple versions of them. Not the most optimal usage I know, but it is very quick and convenient most users (even the not-so-technically proficient ones) are able to use it with ease.