Gruber (implicitly) proves why this is the right move for Dropbox in the space of a few paragraphs.
Many people only use Dropbox as a backup and file share product. That's great. However, it's a terrible business, especially for Dropbox. Backup (and to a large extent) sharing is a commodity product where companies like Google, Amazon and Apple have a massive advantage in terms of scale and in Apple's case, OS integration. Easily moving over to iCloud Drive is exactly why Dropbox cannot build their business around that sort of feature.
You know what isn't a commodity? A single place for all your digital stuff. The big players there are actually disincentivized to build interoperability. iCloud is a product differentiator for Apple, they're not about to build a first class integration for Android and Windows and Google Sheets. Dropbox has the advantage of being platform agnostic. They've spent the last few years building out a dizzying suite of integrations to make this "single place" vision a reality.
So yes, it may not be what you want if you're a backup user, but if you're a business with digital assets scattered across a dozen surfaces and products, this could be very valuable to you.