There's the root of the policy decision. If one device did everything, why would you buy two? Apple needs you to buy the lap AND the tablet AND the phone and maybe also the watch. Oh and they would also prefer you buy more every year, none of this decade stuff.
Or maybe, it's just as Apple says - iOS/iPadOS isn't MacOS and they're not meant to function the same way because they're intended for different use cases and security models.
No doubt they'd love for you to buy one of everything, but your chafing at the limitations of Apple's platform strategy isn't an indication of an ulterior motive, so much as an indication that Apple's platform strategy doesn't meet your needs. Alas.
If you buy an M1 MacBook Air, you spend $1000 and you get pretty much an amazing laptop that does everything except flip over and let you draw on the screen.
If you buy an iPad Pro - the new one with the M1 chip - then you have to spend $1100 to get the 13" model, which has less storage than the MacBook. And that's just the tablet - you now have to spend $350 on a keyboard case to actually use it like a laptop. You can also use a pen input with the iPad, which you can't with the MacBook, but that's another $130 please. So you can really jack up the price of the iPad with accessories intended to give you laptop-like capabilities.
So I don't buy the idea that either device is held back so you have to buy both. The device they're clearly positioning as a do-everything machine is the most expensive one in their lineup.
A) sandboxing, which they shouldn’t change or at best streamline to allow better app interoperability (the current share approach is laughably bad)
B) what is allowed on the AppStore. This is purely a policy issue. Why isn’t there an isolated XCode that at least lets me develop iPad apps? The MacBook Air is a $1000 work horse, the iPad is a pricey web browsing machine/toy. Save for, say artists and even they are extremely limited. Want to create a 3D model using several apps? Good luck with that
For you. But for the majority of the population it is.
Office, Mail, Browser, Photo Editor etc is typically more than enough for most.
This myth that "the average consumer doesn't need x" is pretty pervasive, and it's a pretty diminutive way to look at computing. The user doesn't need a graphical interface, but we still give it to them anyways. They don't need excessive, timewasting animations cluttering their UI, but iPadOS provides in spades.
Either way, I'm expanding my blanket ban on recommending Macs to include iPads. After trying out one of the new iPad Pros, I honestly can't even say it's a better choice than a shitty 2014 Macbook Pro. The ecosystem still has a long ways to go before the iPad feels like a natural part.