It was probably a real threat to them 4.5 years ago. I can't imagine it is now. Dropbox has a large, happy fanbase that's going to keep using it and keep spreading. They're going to keep growing. They've got a small bit of lock-in if you use them a lot too, the annoyance of having to upload a large amount of files to another service. They've got a product that's a joy to use, which is not something Google is known for building outside of their core competency of search.
If I were Dropbox I'd simply view this as market validation (not that they need it at this point) more than anything else.