It’s true that if there’s no competition, a company has no financial motivation to change. And the iPad fills a market with not too much competition—maybe fire tablets and chromebooks take away the most sales. But I believe the theoretical Mac-like tablet would be a different product with a different market, one that is already occupied, solely, with Windows tablets. Again, to me the best evidence that a desktop-like OS tablet may not be intrinsically better for the vast majority is that despite having zero competition, and despite Windows being the most ubiquitous productivity platform in the world, and despite there being many Windows tablet options on the market, and despite Microsoft giving it a lot of effort for many years, I don’t see a lot of Windows tablets in the wild, and I almost never see them used as tablets.Contrary to you, I take a much more cynical view on Apple, which is indeed free to do whatever they it's best for...them.
Which means that in those fields where they don't have much competition, offer the solutions that make the most money for them (for instance trying to avoid overlapping important features that would lead to some people using only one device instead of 2). Again this works only when you are good/unique enough that you can afford to do this, for instance because of a strong ecosystem and products that in some categories are way ahead of (a much more fragmented) competition.
So in short they don't always have to do what's best / most cost efficient for the client, as long as competition is not an issue (and despite what some people say, it's not...).
And you can be at the same time cynical and buy and enjoy their products, even while knowing very well why they maintain certain (often artificial) limitations
To me that says it’s a very small market, and I don’t think Apple or any company can be faulted for not wanting to enter a small market (again, even though I’m in that market). I guess that can be interpreted as Apple not being willing to do what’s best for those particular customers, but again to me that is the more cynical view. I don’t see Apple doing anything unusual. No company is out to minimize profits. So perhaps my cynicism is more evenly spread to all companies.