What you're arguing now is essentially the same theory back then: clones would benefit them by increasing the marketshare for their OS.
That's not what OP is saying:
If the Apple hardware ecosystem were open like that of Windows you'd have major companies all over the world evolving the platform in wonderful ways. Performance would go up and prices would come down.
This isn't "moar market share," it's "the end user would benefit greatly if Apple hardware opened up."
Today is much different than the Mac clone era for two reasons:
(1) Apple clearly has no intention of offering a high-end desktop solution. Instead of losing those users to high-end Windows or Linux machines, they could keep them in the ecosystem by licensing OS X for use in boutique-style desktops.
(2) Apple holds much more power now. In the 1990s they licensed Mac OS from a point of weakness; today they could license to high-end, boutique manufacturers who meet their design standards.
Is this the right way forward? Maybe, maybe not. But OP's suggestion does not necessarily lead to the Mac clone days of the 90s.
That isn't what has happened with Windows machines, though, or Windows itself. It's also the opposite of what has happened with Android: you've ended up with a fractured market full of underpowered phones running obsolete and incompatible versions of the OS.
All this Adam Smith idealism is great until we look at the real world, where the barriers to entry, integration costs, barriers to accurate consumer knowledge and platform compatibility hurdles are too high for competition to actually produce ideal outcomes.
If people wanted a competitive platform with a propitiatory OS they'd all be developing on Windows. There is no reason for Apple to try to compete in the same space, especially not when they are winning everywhere that matters for consumer electronics.
I'll skip the Windows/Google comparisons, because Apple is not them and has never been like them, and get to my main point: I agree!
Apple prints money, and could afford to choose only the cream of the licensing crop (which, to be fair, will be pretty curdled). Here's where we violently agree:
All this Adam Smith idealism is great until we look at the real world where [stuff is] too high for competition to actually produce ideal outcomes.
In this hypothetical I'm not considering "competition." I don't know if this was obvious from my last post, but I'm wondering what would happen if Apple licensed OS X to a high-end desktop manufacturer that would make Mac Pros on its behalf.
Let me be clear: there is a 0% chance this happens. At the same time, it is clear Apple is not interested in the high-end desktop game. So it's fun and interesting to consider the possibility that Apple -- a company with negotiating power in spades -- could pick from any manufacturer on this planet and choose a licensing partner.
That's not what OP is saying:
If the Apple hardware ecosystem were open like that of Windows you'd have major companies all over the world evolving the platform in wonderful ways. Performance would go up and prices would come down.
This isn't "moar market share," it's "the end user would benefit greatly if Apple hardware opened up."
Today is much different than the Mac clone era for two reasons:
(1) Apple clearly has no intention of offering a high-end desktop solution. Instead of losing those users to high-end Windows or Linux machines, they could keep them in the ecosystem by licensing OS X for use in boutique-style desktops.
(2) Apple holds much more power now. In the 1990s they licensed Mac OS from a point of weakness; today they could license to high-end, boutique manufacturers who meet their design standards.
Is this the right way forward? Maybe, maybe not. But OP's suggestion does not necessarily lead to the Mac clone days of the 90s.