Probably worse than zero-sum.
(1) People who were considering Macs could buy cheaper OEM OS X machines.
(2) Sellers of OEM 'OS X' machines have no particular incentive to promote OS X over Windows (probably quite the reverse, given MS's sales tactics) and would probably end up only selling to established Mac users rather than expanding the market for OS X. Look at the wonderful job that Dell etc. have done selling Linux-based machines (they do launch one from time to time, usually more expensive than the equivalent Windows machine and relegated to some obscure backwater of the website).
...don't forget that both Jobs (NeXT) and Apple (in the StarMac era) tried it.
In the latter case, it did exactly what you said - third parties went after Apple customers rather than trying to expand the market.
Clue: Microsoft Windows has a huge list of compatible hardware. Microsoft doesn't write all of those drivers - in many cases, the manufacturers of the hardware handle the support, because of the ubiquity of Windows. Even where hardware can run using generic/standards-based drivers, manufacturers may only test it with Windows (or worse, design it around Windows' gimped version of the standards).
Apple would have to do a lot of work either lobbying manufacturers to support Apple or writing, testing and supporting its own drivers.
Look at how limited Linux's success has been in trying to give away an alternative OS.
Do we get to count Android, the various servers, and the imbedded?
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