Second, systemd troubleshooting and RDP servers are not common uses for Grandma, so even if valid points were made, most users aren't affected.
As to your other point—grandma has an iPad. To the extent Linux has relevance in the desktop, it’s for CS students, people setting up a lab of workstations, etc. Simple stuff like setting up a Remote Desktop is squarely within that use-case.
A random non-expert user would simply not be able to enable scaling properly (and stare at miniature windows).
Yes, I know KDE is better and GNOME is working on this. The fact is that hardware that has been around for years does not work out of the box or even with a 'blah fedora'.
Also, systemd can cause anyone problems - I'm not sure it can distinguish between you or Grandma.
There are no macOS or Windows subdistros. So, the only difference may be between different versions. Adding to that, Apple generally does not have the tendency to replace/rewrite macOS subsystems every few years, so it is much more likely that thing still work after a couple of years.
That's a pretty naive trust in Microsoft's branding/marketing team. Of course there are heaps of different Windows distributions. They don't necessarily differ as much as, say, Ubuntu vs Gentoo, but they do differ.
Something that was working up to a few months ago, the last time I used it.