Yes, potentially someone could by an "Apple" genuine repair motherboard and DIY their own, but that would make a lot of people happier.... then also make best of breed cases, that can only be had with a full Mac or Mac Pro. Yeah, it would potentially cut into their margins on mac hardware for pros.. but they aren't making much money there... the best bet is to keep the pros making content for their mobile devices happy.

Edit: Also, even if they charged a $300 "core" fee that's refunded when you turn in your old motherboard (assuming boards are sold for replacement), that would cover the "OS License" costs for those that are doing it on their own. Apple still makes money, but keeps the third parties to a minimum.