Just because you rewrite some of the slow bits in C, doesn't mean you get don't get a big win using python. I would bet the parts in C don't need to be as agile as all the other parts; the GUI, rest client etc.
So you can iterate fast, and drop to C when you need to.
Yeah, stupid Dropbox, using Python and only having 100M+ users and $200M+ yearly revenue.
Imagine how stupid they feel every morning when they get off their gold plated beds for using Python.