You are assuming the high end of conversion at 5%. And you are also assuming those 25,000,000 accounts are all held by different people, when in reality there are people who hold multiple accounts under different email addresses for different purposes or to avoid upgrading. It's not so easy to switch from one acct to another, probably a good thing on the part of DropBox, but people still do it.

Regardless, it's a great service and if it isn't already a great business I'm sure it will be.