It fails if they don’t buy it.
Designers complain much louder than they praise, and in much greater volume. I wouldn’t equate these criticisms with churn. The aesthetic is ahead of its time. Much like Stripe dictates the look and feel of half the product sites in 2017, I imagine Dropbox is commanding the trend in 2017 with this rebrand (you can already see it with eBay’s redesign right around the same time). I think that’s important because to really play to the design community they have to be ahead, even if that is met with disgust early on.
Of course, I have no idea whether people are dropping them due to this. And without access to their analytics, I don’t know if the rebrand has had a negative impact. If I were a betting person I’d wager that any impact, whether positive or negative, will be short lived. The real test is how they deliver on this brand through features and improvements over the next 12 months.