> but at the same time I do think separate OSes are necessary for radically different form factors.

But the last 10 years have been Apple repeatedly disproving this. iPadOS started radically different from macOS... only to have every major inflection point be defined by it moving closer to desktop/macOS. Whether it was giving up on the single-app model to one that recognized that apps needed to run in parallel (first in a "managed window manager" like a tiling window manager you might find in Linux, and later to a more traditional "unmanaged" one like in normal macOS), to shunning away from keyboards and mice entirely, to then only being an accessibility feature, to now being a major add-on that they sell, and even finally adopting key-commands, etc. (I don't know if anyone ever shook an iPad to undo).

I think this position was defensible at the start, but at the very least has had little supporting evidence since. It is indisputable that the evolution of iPadOS has been to move further and further from iOS and closer and closer to macOS. Perhaps there exists some third way (there probably does!), but Apple has proven to be stretched too thin to try anything other than "maybe it should work more like a phone" and "maybe it should work more like a laptop", and the latter has been the consistent, if begrudging, winner.

> The UX requirements just change too much between phone/tablet and laptop/desktop…

I think this has the line in the wrong place. It should be "between phone and tablet/laptop/desktop". The mistake was starting at iOS and having to trudge to macOS over 10 years vs. the other way around (although perhaps it made sense for marketing reasons). I mentioned this before, and Gruber wrote about my thoughts here [1], but the funny thing about the iPhone is that despite being more constrained than the iPad, I feel less frustrated with it. I am impressed with how much I can get done on my phone, I am understanding when it falls short. But the iPad makes it so clear to me all the time that all that's standing in the way of me getting something done is some designer's stubborn reluctance to allow me to do things the "old way". An easy example is writing long form text before there was a good keyboard you could attach. I get why the phone isn't a great place to type, the iPad feels unnecessary for that to always be the case.

> I can kind of see the larger point you're making – it would be nice to be able to directly use an Apple Pencil on a Mac,

I think the best way to think about this is from user workflows first, as I mentioned above it's not even just about the touch aspects. I think the YouTube experience is pretty great on the iPad. Touch here is kind of a bare necessity, I need to pause the video somehow, but it's not a fundamentally "touch experience", it's just a nice form factor for watching videos on the couch. But that experience immediately changes to frustration if I want to suggest the video to someone or have to quickly reply to an email. I'd love to be able to swivel the keyboard out, copy/paste the link, send it in Messages and type something out quickly. Or reply to an email that's longer than a sentence without thinking "ugh, I should really go to my computer for this". This is a point that is bizarrely absent in all these "the devices complement each other" discussions: they work worse separately. I often want to circle something in a picture and draw an arrow, which sucks with a mouse, then type something out, which sucks with a Pencil. The experience is frustrating on both devices, but would be better if I could do both things. And just like a device supporting all sorts of peripherals doesn't mean its a requirement, you could choose to use it as if it didn't have a Pencil or touch as well. There are tons of scenarios that would be way better even in the absolute worst implementation of this: a machine that when in flipped around in "iPad mode" gave you iPadOS, and when the keyboard was out, have it act like macOS (again, I'm not suggesting this is how it should work, but it represents a "floor" of what the experience could be like). Just the weight savings of not needing both devices is great.

> when you try to fudge both models into the same device you get a mess like Windows 8 or GNOME 3/4 which doesn't serve either set of users particularly well.

I've never been impressed with pointing at Linux at Windows as proof that "the other way doesn't work". It is unfortunate that the cost-to-entry on this hardware means we get very little alternatives to compare. And frankly, Windows and Linux don't do that much great in any UI department, that doesn't mean "if Apple can't do it, it must be impossible." If you only had Windows and Linux, you might be convinced that desktop computing was an unsolvable UI problem too...

1. https://daringfireball.net/linked/2020/04/30/tolmasky-ipad-c...