I'm sure they have better data on this than you or I do. There is an unfortunate tendency on HN to assume people are idiots who don't know what they're doing, as evidenced by every "why does company X need Y engineers??!!" comment.
I'll also admit their enterprise tool that lets you sync only some files as you need them is also seems cool, although I've never worked at a place that used it, and even if I did, I'd probably have to use it in a VM since I've used Linux at my last three jobs.
Dropbox could try to build new products. Keep Dropbox as it is, but then try to branch out and build other tools. Maybe other office tools, or project management tools? They could keep the same login systems if they wants to pitch them to enterprise. But I agree, their core product is sold and should stay that way. Their brand can help them quite a bit if they start offering other products, but only if they're up to the same quality.
I've never needed to get more storage with DB… the free was always good enough though I'm also sure I'm not a big expense to them… many free users are.