This is 100% a concern you need to consider as a possibility. Very unlikely but Dropbox could also experience severe data breaches which would not only likely encrypt your content forever but put Dropbox out of business. In addition, their service could go down for extended periods of time.
You should usually run points of storage with a rule of three points of failure. Therefore, if you have everything in Dropbox you need to have it in two other places. If its for a company you should have a real backup strategy which requires a lot of planning. If its personal data saving it to a combination of external HDs, discs, or your computers storage should be more than enough.