The problem for Dropbox is that they use S3. It's really easy to bypass Dropbox entirely and save money, even if you're non-technical. I use Arq on OSX, and pay only for the amount of data I have on S3 (about $10 per month right now). Dropbox's pricing is a race to the bottom.
It is cheaper to store 100GB in Dropbox than to store your own 100GB in S3. It would cost $12.50 plus transfer costs to use S3. It costs $9.99 and no transfer costs (besides your own net access) to use Dropbox. Dropbox is storing a lot more than 100GB, so they get bulk prices.
The amount of people who are willing to set up an AWS account, generate account keys, download an external tool like Arq, and set it up to use those keys...
No, I don't think that's the biggest danger for Dropbox.
For me Dropbox is all about keeping files in sync between different computers and different people, the backup aspect is very much secondary. Any solution that doesn't do cross platform and cross user syncing and sharing isn't at Dropbox replacement as far as I'm concerned.
For pure backup there's Backblaze, which is much cheaper than S3
