For the most part OP I agree with you, however also need to consider the support life of MacOS that the machine is running, Apple usually does a good job at securing and patching older versions of MacOS however as those OS's age Apple will not be so keen on taking the time and resources away to patch these older versions.
Microsoft had to release certain out of band updates to Windows XP/Vista and 7 to correct a security flaw but it's not guaranteed in the future that they would do it again when the next big intrusion or flaw hits.
Also need to consider the software that runs on top of the OS, Google may cut support to Chrome versions on older MacOS instances thereby making the user go out and finding some other browser that will be supported, same thing for email clients (if used).
Especially now in todays environment people need to make their technology last as long as possible since cost of everything is going up.. but if a user does online banking on their machine or anything to do with finances.. I for one would want to make sure I'm using a device that at least my browser is supported with the latest updates.. if I use a personal financial software like quicken.. I'd like to make sure my OS is patched or still within the wheel house of Apple deploying security updates.
Google drops support for operating systems once they are no longer supported by the vendor. There is a direct correlation between OS support and Google Chrome support.
Google didn't drop Windows XP Chrome support until April 2016, nearly 15 years after Windows XP was released. Similarly, Chrome support for Windows 7 did not end until January 2023, the same month Windows 7 extended support expired, and nearly 15 years after Windows 7's 2009 release.
On the flip side, Google Chrome on MacOS/OS X is not supported for nearly as long. High Sierra 10.13 is only 6 years old and support has already ended.
Since Chrome is the most popular browser, the lack of support and the persistent infobar (every time you open the browser) stating you will not receive updates forces many people to just get a new machine.
So, if Apple provided OS support for 10+ years like Microsoft does, software would continue to be supported, computers would not become "obsolete" as quickly, and it would keep more machines out of landfills.