Monopoly on hardware post-purchase upgradability, repairability, pricing relating to components offered on hardware that is legally only permitted to run on Apple produced and sold hardware. That is a monopoly relating to a product cycle. In comparison other manufacturers use industrial standard COTS parts that can be upgraded or repaired without going through that specific company and you do not have to purchase a warranty from those companies either as they will sell you genuine parts.
I believe you are thinking that official macOS users have a 10% marketshare, I am not sure if this is accurate as Windows and Linux runs on servers, POS, etc. I remember the consumer marketshare being around 3-6% for macOS and I wonder why that is because one the OS is not licenses and only included with the purchase of Apple hardware and two the cost of that hardware is absurd expensive for what one is getting in comparison to other manufacturers.
It is possible to run macOS on non-Apple sold or supported hardware for educational purposes but nothing more as it it becomes a legal issue. Years of free updates is questionable as some Apple hardware is considered obsolete or vintage due to parts availability however is artificially bared from running the latest macOS, even third-party hacks have proven the official unsupported hardware can be run, it is Apple who limits support as to persuade those users to purchase new hardware. Windows and Linux for example can run on hardware for 5+ years. The usually limitations of not having more than 4GB of RAM to run a 64bit OS and apps or more than 128GB of SSD has been long passed as most users usually use they laptops for office environment work, the few that do require more computing power care more about the software used vice macOS additional features. As mentioned desktop OSes have matured a long time ago, hence the focus is on mobile OSes and the incorporation of its benefits on a desktop model.
I have had iMac's, iBooks, MacBooks, MBPs and MBAs, I have never run into a situation where I could not complete any photo editing, processing work or video/music editing work. Granted I am not editing more than two 4K streams at the same time on my MBA, but it is still capable.
Most users over purchase a product rather purchase what they need.