As someone with plenty of C++ knowledge on mobile, I would expect there to be more C++ devs available with good mobile skills than there are rust devs?

Maybe my own viewpoint is twisting the amount of devs I think are available, but I don't think the choice dropbox originally made was crazy. C++ is a great way to share complex code over multiple platforms (i.e. desktop + mobile) if you're running a relatively small engineering team with limited resources.

Now, if you have semi unlimited resources, it makes sense to rebuild things natively for each platform in many cases. But Dropbox might not have been around, had they not maken the choice to share part of their codebase cross platforms.