How else do you think Drew Houston was convinced that this design was the right step for them?
I'd also like to point out that not all redesigns go as planned, but iteratively eventually 'get' there.
See: Android's honeycomb and later Ice Cream Sandwich redesigns before they settled on Material Design.
One of the few apps to have gotten design different and right, at the first time of asking, have been, IMHO, WhatsApp, Stripe, and AirBnb.
