I think the majority of Apple users are casual enough that the differences don’t matter as much. If Apple is banking on people buying two devices for different needs, I’m skeptical about how well that stands up realistically. Meanwhile people like us on a computer forum are not the casual userbase and our needs are probably different, and we’re more likely to notice the differences
If iPad OS doesn’t need full functionality, then it at least needs most functionality. I think making a pro device live up to the name and actually function like a pro device should be something that would drive more sales, because then there are even less differences and all it would come down to is form factor (and people will buy two devices for different comfort levels) even if it‘s not 1:1 equal to Mac OS…
Apple could clean up if they made a pro version with a full OS on it, I'm thinking of Windows users such as myself who would most likely switch. I cancelled my iPad order because I know that iPad OS will never replace Windows for me, at least not in its current iteration. I'm also willing to bet that a large percentage (if not a majority) of iPad users also have another computer, whether it's Mac OS or Windows. Giving the iPad a real OS might not cannibalize their own market, on the contrary I think it would actually bring in many of those Windows users, and there are a crap ton of Windows users.
But yeah, I don't disagree that Apple users are casual enough to accept iPadOS, the vast majority of their interaction is simply consuming content and the iPad is just fine at that. A lot of it is marketing though. A surface pro is just as good of a "dumb" tablet as an iPad, they have certainly closed the gap in terms of things such as battery life which the iPad used to reign in, and has the added benefit of not having to compromise on the OS on such things like user accounts, multi-tasking/backgrounding, etc. But Apple is just a hot ticket company and people want their products, that loyalty is hard to break. I know it well, I'd MUCH rather be on a Galaxy Fold 3, but have to deal with an iPhone because all my friends, family and work associates are on one.
