You make all legitimate points. I would say the following in rebuttal:Apple tried that once in the 90's, it... didn't go very well. One of the first thing Jobs did when he came back as CEO was kill off the clone program, since the clones were mostly siphoning away hardware sales from Apple instead of bringing new users to the platform.
Regardless, I don't think there's a pressing need for Apple to expand its market share: 15% is way bigger than it used to be, and in the Web App era there's increasing less correlation between market share and software availability. Apart from AAA games, the main areas macOS suffers from lack of native software support are specialized industry applications and drivers (e.g. Solidworks in engineering), where people are ususally buying higher-end hardware anyway.
Also, one of the huge things Apple achieved with the M1 is incredible vertical integration: instead of having to support all the quirks of dozens of different 3rd-party CPUs and GPUs, they now get to control and design almost all of the hardware to work as nicely as possible with macOS. Opening up macOS to third party manufacturers would mean adding a ton more complication to their software testing and support.
(1) I know the history but the cloning program of the 1990s was incredibly mismanaged as Apple (which was teetering on bankruptcy anyway) lost money on the licensing fee and the clones, many would say, were superior to the computers made directly by Apple (neither would be true today);
(2) the OP asked for ways to Apple to increase market share as a means to increase native software support and development (not whether it needed to do so). Even still, I believe that increasing market share is, in fact, absolutely in the interests of any company for many reasons but especially so in the case of Apple, which is very reliant on its iPhone business; and
(3) vertical integration need not be sacrificed with a tightly controlled licensing program to one or two manufacturers (I am not talking about an open licensing program to everyone) that uses propriety chip sets manufactured by Apple.