I'm guessing this isn't something that is easily determined, for any platform. Microsoft can guess, at best, what kind of revenues there are for third-party so

I'm guessing this isn't something that is easily determined, for any platform. Microsoft can guess, at best, what kind of revenues there are for third-party software. They have no idea. Apple, they can only speak authoritatively about their own App Stores, but they don't give out these numbers. Linux, well... it's Linux. My guess would be that revenues are smaller for this platform, because hardly anyone charges for their stuff. Actually, I wish more paid work would be done for Linux. I'd be happy to pay for good apps.