I think they'll be fine.
Yes, only 2% of customers may be paying - but customers pay to get more storage.
More interesting stat would be how much of their storage is paid for?
How much of a paid storage level is actually used?
Their conversion rate of free to paid?
Dropbox was my first 'cloud file thing' and I have a soft place in my heart for them for that free service I really appreciated.
I semi-randomly ended up paying google a tiny amount each month instead - but I think dropbox is still the one that's going to make money - and my reasoning is because my mother uses it.
Not just that she uses it, but how Dropbox doesn't intelligently de-dupe files, but how it encourages duplication across accounts.
e.g. My brother-in-law creates a folder containing pictures of his children and shares the link. My mother clicks on the link and mounts it to her dropbox account. She loves that, a folder where pictures of her grand-children magically appear - but for every photo uploaded, it consumes her free dropbox space - and then ultimately nudges her to pay (just a little bit) - whilst on the server side the cost to dropbox is just a smidge of additional metadata.