Apple has a de facto monopoly on hardware allowed to run macOS. This is not in the best interest of consumers because it removed competition from this space.
This point has been argued ad nauseum. Not just in the Mac community, but in the business/financial community as well.
While Apple has total control of both Mac OS hardware and software, it does not constitute a market. There is no actionable market for Mac OS. The market is PC operating systems and Mac OS does not constitute anything close to a controlling share of the market. Therefore, anything that Apple does, doesn't violate an abuse of a monopoly powers.
An analogy (not perfect) would be that you can only get Big Macs at McDonalds restaurants. McDonalds has 100% control over Big Macs sold around world. However, there really isn't a Big Mac market. The market is fast-food hamburgers and there are plenty of alternatives, but none of them are Big Macs. Would there be more competition if Burger King franchisees were allowed to sell Big Macs? Sure. It would drive competition and be beneficial to Big Mac customers if they could go to a Chick Fil A to buy their Big Macs.
Mac OS is a niche OS. Apple can offer or not offer it to other hardware vendors. Apple chooses not to.
Anyway, you have to be careful in defining a narrow market. If a market is defined too narrowly, there could never be any exclusives.