Note that in all of your examples of retina transition, the screen size remained the same between the non-retina and retina version. The iPad resolution is not a multiple of the iPhone resolution.
When Apple used 2x, it was so that developers wouldn't have to make major changes to their UI code as iOS took care of multiplying legacy coordinates by 2.
It worked because the PPI was also increased by the same proportion, keeping UI elements the same physical size on the screen.
What you're proposing is doubling resolution but not PPI resulting in a bigger screen size, which negates the advantage of not having to redo layouts.
It would result on a different, non-multiple minimum size in pixels for touch targets and buttons which would mean unprecedented work for iOS developers.
As of now, a dev can use the same exact button size across all iOS devices (doubled or halved automatically by iOS depending if they use virtual or real pixels in their code). I don't think Apple will want to break that when 95% of users wouldn't be able to notice the increase in PPI anyway.
Simply keeping 326PPI and increasing the resolution to make a bigger screen would mean much less work for devs than what you propose.
One does need to take into account that if apple is considering to keep its ongoing practice alive (Basically one phone structure, screen etc for 2 years and changing the specs) then your 326 Ppi screen is going to have to last you 2 years specially when 498-500 PPI LCD's are already advertised in the market (Sharp) and when your main competition in the high end smartphone domain is going to launch a 2k display with a 500Ppi before you. I predict that by the end of 2014 Samsung (atleast them) would have atleast 2 2k displays with 500+Ppi (Galaxy S5 and Galaxy Note4)..And this time they are seriously considering a High PPI LCD version as well (probably based on the Sharp LCD with IGZO 498 PPI displays)....I know apple does not go out and out with spec comparisons, but they can also not totally disregard what is going on in the display world especially when they like to lock their display tech for a period of 2 years. If they are to bite the bullet and work with devs to bring the neccessary changes then i would rather have them go all out and establish a new benchmark for themselves for display tech and keep it for 2 years as opposed to starting out with something that has been done a few years ago and keeping it for another 2 years.
And before we bring the entire quesiton of power requirements for bigger, more powerful screens, we do need to remember that Chips have become more efficient, battery tech has advanced in 2 years and a larger iphone will spot a bigger battery.