Speculation: momentum of people who were early on the cloud adoption curve who are implicitly/behaviorally entrenched by accrued data volume. That doesn't sound to me like the foundation of a solid business with growth potential (see: IBM). Those kinds of users are ultimately going to be fairly price sensitive given the substitutability of the product and therefore not marginally monetizable. They may already also perceive themselves as paying an 'inconvenience penalty' vs GDrive/iCloud.
Honestly, this looks like exactly what it might very well be: rich Silicon Valley investors dump a squeezed, has-been company into the frothy public market to tap into 'dumb' liquidity.
I should clarify I don't have anything against Dropbox itself. I do have a problem with pushing a dud into the public market at an irresponsible valuation to effectively defraud uninformed people and desperate institutional investors with low accountability who are facing a drought of good return opportunities (e.g. mutual fund and pension fund managers). You can hem and haw around it all you want ("oh they'll build new features", etc), but the growth expectation baked into their valuation is more or less predicated on extrapolating a pattern of success they are unlikely to be able to replicate. Cloud storage is commoditized and they're competing against monster platforms that are better positioned than them to capture new business and siphon their existing users (applies even in enterprise, arguably they lose against Box but certainly against Microsoft and now even Google). So they're being paid to try to tap into new verticals as very late contenders (e.g. Dropbox Paper)? There's no there there. It's wrong. In my opinion, it's fraud and only slightly less awful than the behavior surrounding the dotcom bust.