As you say, we're getting exactly these advances with phone and tablet OSs. iPads are becoming a dream appliance in the ways you mention and capture 90% of the average person's computing needs.
Regular computers have to maintain some backward compatibility and it's nice for power users to still be allowed to fiddle with file extensions and system internals.
Correct. Or they become Netflix viewers, or are used to doodle in Procreate or something. I am yet to meet a person who uses the OS (as opposed to specific apps) for something complicated. Apple loves to show how easy it is to drag an image from Email in to Pages. But nobody uses an iPad for Pages. And even that simple interaction is still somehow easier on a Mac.
The latest MacBook gives me hope that post-Ive Apple is serious about giving their customers what they want. I would love to see an iPadOS for people who do actual work one day.
Note: I'm trying to make a distinction between iPadOS and the apps on an iPad. Plenty of apps are useful for actual work. However the OS itself is very much a hinderance.
People who want to use Pages in iPad... for the sake of your life, please don't do it. It is quite... aggravating trying to move that freaking table from few cm across and it ended up 3 inches away. My forehead vein was ready to burst that day. It is not worth the experience to use it on the iPad unless for a simple essay or letter which should be fine. But if you try to go beyond that, make sure you remember to go to your happy place so often when you try.
"OS, not apps" is a kind of weird distinction for a lot of users, though. I used an iPad as my only portable computer for a bit over a year, so I get what you're saying -- there were definitely things that were easier to do on macOS than iOS, and those things tended to revolve around system-level features that were present in AppKit but not UIKit. But it was also surprising to me how little there was that I couldn't do on the iPad.
HN users tend to understandably be focused on development tools, where iOS really is pretty handicapped. (I suspect this is only going to change through moving development environments fully into the cloud. But there are an awful lot of people who use computers for an awful lot of things who probably could replace those computers with iPads already. (Do keep in mind that one user's "doodle in Procreate" is another user's "bring in several thousand a month on an art Patreon.")
I had an android tablet at some point and... it was nice to read comic books on it I guess ? But whenever I wanted to do something a bit productive - I remember trying to do mindmaps, bibliography work and similar research-ey stuff it was painful and I ended redoing everything on a computer with keyboard shortcuts. On the other hand my Remarkable is seeing a great deal of work... Drawing little schemas on it is incredible.
Regular computers have to maintain some backward compatibility and it's nice for power users to still be allowed to fiddle with file extensions and system internals.