In a nutshell, the move is an effort by Apple to prevent any possible exodus of app developers to Windows, nothing more. Apple's intent was to consolidate around 2 Mac models - the MacBook (pro) and iMac (which make up the bulk of their sales), while retiring the Mac mini and Mac Pro.
You're speaking in certainties, as if you were there in the board room when this was discussed and decided. It IS something more, and there is likely a bunch of reasons (including yours) why they've decided to 'double down'. I'm not an app developer, but I'm a Mac Pro customer. What about the photographers, designers, scientists, video pros, 3D artists, audio engineers, CAD designers, educators who might also fit as the target demographics? Phil alluded to the very broad array of customers for a computer like this. Are these not worth trying to hold onto, and perhaps grow? And what about the growing area of VR? Palmer Luckey said that if Apple ever released a decently specced computer for VR, then Oculus would support it. Gaming could be a huge growth market for Apple if they released decent hardware and updated some of their APIs. In terms of numbers, the Mac Pro's would indeed be higher if it was a more attractive option for it's class. It would never reach MBPs or iMacs, but to be fair it's been crippled, so no wonder the numbers are low.
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