Show me a business that is willing to offer you that guarantee and I'll show you a business that is out of business.
What if there's a fire? Or malicious destruction? Or massive hardware failure?
Anyhow, the only people who would down-vote some of the comments I am making are those who have never suffered the pain and agony of data loss. If you've ever been in those shoes you know, without an iota of doubt, that what I am saying is 100% on point.
There's nothing wrong with Dropbox. The problem is in how naively people are choosing to use the service.
Could they be better? Of course. Would I still have my own redundant backups and recommend that Dropbox users do the same and fully understand the service. Absolutely!
Personal responsibility is a bitch. It's much easier to blame someone else. Guess what? Blaming someone else for your failings will not make things right or bring back your data.