Someone's going to try it at some point, and run smack into the main problem: Dropbox will just ban them based on the acceptable use policy as their upload gets ludicrously large. To sign up you also have to pay a minimum of $1200 (5 users for 1 year) and submit proof of your status as a business. They don't let you do unlimited personal anymore.
Backblaze is another unlimited-ish option, but the problem there is that restores are glacially slow, even for one file. I wouldn't be surprised if cold data on Dropbox was treated the same way.