That's not the point, especially when wifi is available almost everywhere
We are talking about iOS being close to identical on devices with dramatically different screen sizes. When the 5.5" iPhone 6 launches it's going to hurt iPad sales quite badly, especially the iPad mini
I don't think a substantial number of iPad users care how well-optimized iOS for iPad is - screen dimensions undoubtedly trump that. I also think that, though there are undoubtedly those who want to see
more differences/optimization between iOS iPhone and iOS iPad, there is probably an equal group that want the experience to be as similar as possible. And then there's probably a much larger group that doesn't really notice or care one way or the other.
I don't disagree that there will be
some people who decide, "The big iPhone is big enough, I don't need to own both any more." I also don't doubt that, among iPad models, the Mini will probably be affected by that more than its larger siblings. But of those who currently own both iPhone and iPad, a larger iPhone is not likely to change the fact that they prefer to do some things on a screen
the size of an iPad. Perhaps they'll use their iPad less and iPhone more. But that doesn't change the basic proposition. For viewing, bigger is better, for portability, smaller is better. Which factor is more important will vary from person to person, and from moment to moment.
But what really matters to Apple is not whether they lose some iPad sales, but that, overall, having a larger screen captures more new $900 Apple customers than it loses in $400 iPad sales. That's no different than iPhone's "creative destruction" of iPod.
Though the pace of adoption has slowed for tablets, the tablet market has a long way to go before it saturates. As long as Apple can find as many iPad customers as it loses (and the IBM deal is one example of how that can happen), iPad is not likely to be an endangered species.