Is Django a big enough project for you?
Did you know that Django was not only successfully migrated from Python 2 to Python 3, it was ported in such a way that for many years it used the same codebase in both languages ...
Perhaps that's the biggest advantage of porting from 2 to 3. A lot of the code could run in both languages.
Is Dropbox open source? Is Dropbox even a typical Python application representative of the challenges of porting from 2 to 3?
My hunch is that the challenges of porting Dropbox to any other language have to do less with Python more with the need to deal with a filesystem at a lower and more granular level than what typical programming languages offer. Thus everything needs to be rewritten in bazillions of ways to handle the bazillion corner cases.