You have hit on the real issue here, in my opinion, but framed it terribly. The suitability of Linux for any given person often has nothing to do anymore with ease of use or of its capabilities.
As with others here, I have people in my life I provide tech support to. I have moved some off Windows to Linux and had those requests go down. There are others I would never recommend Linux for. My wife uses MacOS.
I use a bit of everything operating system wise. My most frequent daily driver is my wife’s old iMac but I run Linux on it. By default I would have left MacOS on it but MacOS was a massive pain actually. Many of the applications I want to run are a hassle to source and maintain on MacOS. A bigger problem was that the iMac no longer runs the most recent MacOS which means there are many other apps whose current versions cannot be run on it either. For me, moving to Linux on this hardware meant dramatically fewer problems, more focus on actual work, and easy access to a much larger and up-to-date universe of applications. I do not mean just Open Source either. Examples of applications whose latest versions only run on Linux include Microsoft Edge, Teams, Zoom, Slack, and probably a bunch more I have forgotten. I use a lot of Podman ( Docker ) containers and these work significantly better under Linux. I collaborate professionally and share office documents every day using both LibreOffice and Office 365 on the web. To counter your example, peripheral compatibility including my newer printers and scanners is better under Linux as well.
I honestly doubt I represent a less “demanding audience” than your parents. The percentage of the market that would be perfectly happy with my setup is probably not “narrow”.
I have no doubt though that running Qhickbooks would be a hassle for me.
And this I think is the key point of the “normies” argument.
My mother does not have demanding needs. What she does have are friends that will recommend things or send her stuff that assumes the same environment they are using. Her “techie” friends ( hardly ) may send her instructions on how to do things which again assume she is using the same environment they are. Unless you are a tinkerer, all this is just easier and goes better if you are using the same platform ( Windows for almost everyone ). The same could be said of Chrome vs Firefox though really. People buy iPhones so that their text messages to friends show as blue instead of green. For many people, the killer feature is “being the same”.
If you are a “normie” and you heavily rely on an platform specific application, switching is hard. Not because other platforms lack power but simply because they are different.
My wife’s friends and colleagues mostly use Macs and, because of her iPhone, she is a heavy user of the Apple ecosystem. MacOS makes the most sense for her.
If I completely switched back to MacOS or Windows at this point, I would hate it and have a long list of examples of basic tasks that are too hard or how they are not powerful enough for me. Many people who switch to Linux feel the same.
As above though, I use all the major operating systems regularly and, as a casual user ( what I am much of the time ), they are all fine.