That last comment is ironic, given that Dropbox would literally not exist today without the runtime patching of the Finder.
There was no other way to do what they did. The only reason Apple added API was because Dropbox came up with an idea that demonstrated the need for such an API to exist.
Their filesystem integration would have been just as seamless without the badging. I admit the badging is a useful feature, and certainly made Dropbox better but by no means is it core to the product.
You're talking past each other — other, more fundamental features used to require hacking OS X.
See for instance https://news.ycombinator.com/item?id=12467149. IIRC ArsTechnica wrote at some point that monkey-patching Mac OS/Mac OS X was standard on the platform.
There was no other way to do what they did. The only reason Apple added API was because Dropbox came up with an idea that demonstrated the need for such an API to exist.