Kill the niches, save the world? Huh?
"Why should I carry both an iPad Pro and a MacBook Air, when it's nothing more than a wasteful duplication of hardware resources?"
That is a personal choice. Choose not to do that. I have a MacBook Air M1. I do not want and would not use any iPad. The closest device I have is my iPhone 7, which is a workhorse within its niche of purposes, few of which would be practical on my MBA M1.
Therefore, I don't understand what you're talking about. Your point is pointless to me.
"The security issues of a single machine running two OSes are real – but they could be managed. The resource issues, however, will continue to plague us until we operate within an understanding that general-purpose computing devices aren't simply landfill-in-waiting. Nor are they meant to be tied to any particular set of tasks at the whim of the manufacturer."
Obviously, the M1 and M2 Macs could run iOS or iPadOS or watchOS. If that fits one's needs, then do it. But the compromise would have to be isolating each OS such that the wide open source of apps and services on any Mac could not breach the not-Mac sandboxes. This is well done already, if desired, using Parallels to run Windows or any other compatible OS. So if that's what you need, it can be done.
Do I personally need this? No.
You call Apple's fulfillment of niche products "the whim of the manufacturer"? Why? I have no comprehension of what you're talking about, once again. The distinct niches of each of these products are painstakingly chosen as well as successful.
As such, this statement makes no sense from my perspective:
"...Many have openly wondered why the iPad has not had an option to run macOS."
No. That would be incredibly hard and nonsensical specifically because of the niche of each of the to products. You work with both and think it's possible to run full MacOS on an iPad within that hardware and graphical user interface realm of simplicity? Nope. That would be a nightmare on several levels. The Mac is the far more able device, allowing for vastly more uses than any iPad or iPhone or Apple Watch. The Mac does not, and must not, have a touch screen. The reason why is the "Gorilla Arm" effect, proven many decades ago, whereby no one wants to have to reach up to play about the screen with their hands specifically because of the muscle, tendon and ligament agony that rapidly results. Apple is, despite many blunders, at least wise enough to stay out of the foolish realm. Therefore, moving iPadOS, etc. to a Mac is not practical unless the interface of that touch oriented OS can be easily and comfortably move to the touch pad, which I personally find to be far preferable to the relatively ancient mouse interface. I never use a mouse at this point.
And so forth. I could rant on about further points in the opinion article. But it all comes down to what YOU, that being any particular user, want or need to do with your computing device. Limiting the variety of devices makes no sense at all when considering the diversity of uses of such devices. Take into account diversity and the focus upon usability for each function among the entirety of users. That's what Apple is doing.
AND Apple is minimizing what ends up in a landfill at the end of life. If Apple is really doing that and continues to do that, then that is the best for all concerned. Worrying about your need to use both an iPad and a MacBook Air? That's a YOU situation for YOU to deal with. Consider dropping one and only using the other. I'd suggest the far more powerful MacBook Air. But by no means would I limit you to only the MacBook Air, or the cruddy niche compromise that combining the two would create.