What makes Apple valuable is precisely its full vertical integration. Nobody else does it at that scale except maybe Sony for consoles and Microsoft but both don't have the same usage coverage and thus integration.
It wouldn't be economically rational for Apple to bring down its own moat.
People whom want Linux on Macs obviously already don't want to be put into Apple's ecosystem.
Apple isn't Sony. They make an arm and a leg on their hardware.
When they switched to x86, it didn't take long for them to release Boot Camp, so they could take Windows users' money, too.