Were that so, we wouldn't fix them, we'd just replace them with Macs.
No you wouldn't. Have you priced any parts for macs recently? Talk about running into trouble with a particular motherboard model not made anymore, what do you do with the imacs with the GPU solder problems? Replace the board with an identical model, with the same flaw? Yeah, great.
Need a new LCD on your macbook? Oh, look, Apple decided to take a standard LG lcd and reverse 8 of the wires so that you have to buy an Apple LCD from Apple for $400 instead of the generic $60 LG model.
$250 for a new license when replacing an $80 motherboard would still be better than paying $500 for an Apple motherboard. Shit, those things frequently cost more than $330 used on ebay.
Any circuit design must contain at least one part which is obsolete, two parts which are unobtainable, and three parts which are still under development.