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I don't even think it's true that a high proportion of developers use Macs. My guess would be that most developers use Windows, because most developers work for corporations and most corporations issue Windows machines to all employees. Companies which offer a choice are in the minority.

Developers drive adoption (not least by creating software) and high specs.

Yes, but those developing for Apple OSes would have adopted whatever OS Apple used.

The developer who pick it because it is a Unix are people who developer for Unix like OSes, mostly for servers, so they mostly do not produce software that drives adoption by people who are not developers.

This is true. Only upside I can see is that the developer tools available for "unix-ish" system were really strong (if clunky at times).

I think you'd have got a much smaller eco-system of third party apps for MacOS (think omnigraffle etc). Whether that would have made a substantive difference I can't say.