I agree with this but I would go even further and question how Apple would introduce a larger tablet when the current 4:3 aspect ratio becomes less ideal at increasingly larger sizes and there is no simple upscale that would allow for an easy transition for developers, unlike with the iPad Mini's simple downscaling.
Good point, but it would make it soooo much easier for devs if they could use the same set of buttons/bitmaps and touch target and font sizes across all iOS devices.
Also, showing iPad (mini) apps on a 12.9" would make them look huge and clunky so this would only be a stopgap measure anyway, much less important than keeping consistency for developers.
The iPad resolution was not a direct multiple of the iPhone resolution and while the density changed, it got lower so touch targets getting a little bigger physically was not a problem requiring resizing of those elements.
How about 3072x2304 at a more reasonable 298 ppi? Non-retina versions of unoptimized iPad apps could be shown at 3x full screen while avoiding fractional scaling. Not ideal but exactly what Apple did at first for iPhone backward compatibility on iPads.
And the fact that iPhone apps on iPads didn't look good certainly helped accelerate the development of native iPad apps.