I appreciate the lengthy answer, but it builds on the premise that I can't possibly know what I'm talking about because otherwise I would've come to the conclus

I appreciate the lengthy answer, but it builds on the premise that I can't possibly know what I'm talking about because otherwise I would've come to the conclusion that code sharing just sucks. My point with engines still stands: they share huge chunks of codes. Even code that has nothing to do with low level abstraction (animation, loading models, mixing audio etc.). And yes, their job is to run on all their target platforms. But if your app targets multiple platforms that's your job as well. What you described with spotify is that you were essentially building your own phonegap. That might be the right answer if you want to achieve as much code sharing as possible. But just because you can doesn't mean you should. You can't go ahead and make wrong abstractions and assumptions about platforms and then tell me I shouldn't even be trying because you failed at it, when others somehow achieved it. And I know several non-SV companies (a Navigation vendor I worked for for example) that ported their C++ code base to mobile in a matter of days. So it might as well be possible that the engineers in charge at Dropbox, FB etc. were lacking experience.