You can see why I believe Apple is going to regret not making OS X available on iPads by reading some of the points in John Gruber’s iPad Pro review:
First, when the iPad Pro is open with the keyboard attached, holding your arm up to touch the screen for anything longer than a moment or two is ergonomically uncomfortable. Apple has stated for years that this is why they don’t make the displays on MacBooks or iMacs touchscreens (that, combined with the relatively tiny click targets of Mac OS X, which are designed for very precise mice and trackpads, not imprecise finger tips). Scrolling through a long document using the iPad Pro touch screen is uncomfortable when it’s in laptop position. Going through a slew of new emails, likewise. In laptop mode, I want to use the keyboard for these things — and in most cases, because of bugs and/or software limitations, I can’t.
I don’t know if John has ever used a Surface or even a Windows 10 device so I do not know what he would think of them. However, the fact that Windows 10 can switch between Tablet mode and Windows mode is exactly what makes it work better in these in-between situations and on these in-between devices. Devices that are both touch capable and can have keyboards and mice, I mean. Windows 10 handles this exact situation far better than iOS or OS X do today. And as someone that uses an iPad daily I long for the ability to switch between iOS and OS X on a single device.
Don’t take my quoted bit out of context. Read his entire review.