Your definition of a PC being made of OEM parts is wrong.
That's not my definition of a PC. That's the opposite of my definition of a PC. All your subsequent arguments are thus invalid.
But by definition a PC is a computing device used personally (as opposed to a mainframe or other computing formats) to run a variety of tasks from productivity to entertainment.
That's the Wikipedia definition, I guess, but it's not very useful...you could capture programmable calculators under that definition. The market definition comes from the old "IBM PC compatible" label, which distinguished "PC" as a whole class of computers that were essentially interchangeable, regardless of who specifically manufactured them. (Early on, it didn't even matter who programmed the operating system...not all versions of DOS were by MS). That's part of why "Mac or PC?" was a meaningful question. More recently, that flexibility has extended to operating systems. Uninstall Windows and install Linux, and it's still a PC. These days, Macs are in a gray area. I would argue that since switching to Intel and opening up their ecosystem to the point where you can install whatever OS you want on the hardware, the distinction is really "Windows or OSX?"
The windows PC won't stop being a PC if MS decide from Win 8 onwards you can only sell your apps through their storefront.
It would if it was impossible to uninstall Windows (without modding the hardware or some kind of software "jailbreaking"), which would also entail only being able to get Windows on specifically MS-licensed/designed machines. If the operating system, hardware, and software are a closed ecosystem, it's no longer a PC. It's something else, some kind of specific, dedicated thing. I don't know what you'd call it. It's also why if you built a high-end gaming PC and installed nothing but games on it, it still wouldn't be a "console."
The difference between a Sega Master System and a ZX Spectrum was the purpose to which the hardware was designed and engineered.
You also couldn't write your own software on a Sega, or distribute software without getting a fancy cartridge from Sega themselves.
Edit: If it's just an interpretation of PC that matters, one can replace my reference to PC in earlier posts with GPCD (general purpose computing device).
Okay, then I would classify a console as a MPCD (manufacturer's purpose computing device). If I can't do as I damn well please with the box because it is specifically designed to lock me out of access to its processing hardware, it's not a "general purpose" machine. Consoles are still that, no matter how many non-gaming applications you can buy for them.