Can't we just buy a $10 stylus right now for the iPad?
Yes, but the $10 ones are just finger extensions and while they might give you a bit more precision than a finger, they're not a patch on the styluses on Samsung and other tablets, that offer features like:
[*] Pressure sensitivity
[*] Stylus buttons with extra functionality
[*] The ability to distinguish between a hand touch and a stylus touch - so, for instance, it can ignore the users palm brushing the screen while they're drawing.
[*] The ability to 'hover' without clicking.
To get this you either need an "active digitiser" built into the screen (as per Samsung) or a smart stylus in communication with the OS.
Steve Jobs: If you need a stylus, you've already failed.
...and that was quite true in the case of the original iPhone.
NB: No multitasking and a tiny screen also made sense at the time on something that was still, primarily, a phone. Things moved on.
A 12" iPad "pro" is another kettle of fish - it is
not going to get used in the same way as a smaller iPad, let alone an iPhone. It will be much more of a desktop-bound device, and appeal to people doing serious graphics or making extensive use of handwriting recognition. Its going to be in competition with things like this:
http://www.engadget.com/products/wacom/cintiq/12wx/
Personally, I'm a bit skeptical about the whole iPad Pro idea, but a 12" iPad without an active digitiser (or a smart stylus with similar capabilities) would be a complete non-starter.