Who has successfully managed this kind of transition? The obvious case is IBM which is now essentially a consulting company and doesn’t sell PCs anymore.

Apple's entire success story is their vertical integration. They can't do that and OEM.

As for the PC makers: they don't innovate. Microsoft doesn't care who sells PCs, Intel doesn't care who sells PCs. Every PC maker is essentially an assembly company. If you appreciate Apple's innovation in the laptop space over the past x years, then you don't want Apple to be an OEM.