That's true...However, I can't imagine one of the biggest tech companies (with tens of billions in the bank) won't be able to make iOS multi-user. iOS was built off OS X and Apple already did this for OS X. I guess we'll see if they actually want to do it this fall, as I imagine they would do it with the iPad Pro immediately or not do it at all.
Oh, I have no doubt that Apple
could add multi-user functionality to iOS. However, I think in doing so they would need to increase the price point of iOS devices (can you imagine the uproar this would cause?) due to the increase in hardware and system resources.
Apple also need to come up with a way to avoid alienating current device owners, I seriously doubt any current iOS device could stand up to multi-user functionality without some kind of degradation in user experience. New multi-user devices will also be scrutinized by the various mobile carriers for security reasons.
And, as bigjnyc pointed out, this would likely result in lost sales as relative users migrate to multiple users on a single device.
As far as the iPad Pro, I don't think that's going to happen anytime soon. Designing a larger iPad is going to bring with it a need for new materials/manufacturing methods. It's going to be quite difficult to design a larger iPad, say 12 inch, without also increasing its thickness and weight. Otherwise we'll be staring another round of bendgate square in the eyes.