Sorry but unless I'm misunderstanding you, it seems that you have it backward.
Not backwards, we just see the problem differently.
I do see your point though. You are thinking that DPI must remain the same as a way to keep UI size the same, whereas I am thinking that resolution must remain the same (or 2x) to avoid fractional scaling of bitmapped graphics.
I was thinking that DPI is not that relevant as long as touch target size is no smaller than what is currently on the iPhone. (DPI ≤325)
If they simply scale the display size up without changing the resolution (or moving to 2x) all that happens is that items become larger and easier to use.
Keeping resolution the same and going from 4" to 4.7" would be similar to the difference in size between items displayed on an iPad and an iPad mini. (actually, the difference would be slightly less)
They clearly had no regard for keeping DPI the same when they released the iPad mini, as they kept the resolution the same (and thus increased the DPI) because touch targets--at least when following Apple's spec--should have remained similar to those on the iPhone.
Personally I find that many objects
are too small on the mini though, as most app UI's were designed for the larger sized iPad.
With the changes that have been made to iOS to accommodate the iPhone 5, it does seem possible that the UI of newer apps could simply "reflow" to fill the screen if the resolution is increased. (rather than displaying 4x5 on the home screen, you could display 5x6 or increase the gap between icons, for example)
I don't know if the changes they made were simply to add an additional height row, or if they were more forward-thinking and would allow currently existing apps to reflow and fit most resolutions.
As long as the pixel density remains at or below 325 DPI, this would mean that they could increase the resolution as a way of providing more "workspace" without requiring the UI to be scaled - though it would mean that you have apps which don't fill the screen if they don't support this feature. (similar to iPhone 4 apps on an iPhone 5)
However, I don't see why a "4x" retina display is so infeasible. There is talk of 2560x1440 phones already.
A 4x retina display would be 2272x1280 - and that would look good with
any size of phone.
I'm going to guess scaling instead of letterboxing. With a retina screen, scaling shouldn't be jaggy or blurry, would it? And the graphics processor could probably handle it well.
Scaling is horribly blurry on the Retina MacBooks, text has to be displayed at 1x or 2x resolution to look sharp.
And scaling by small amounts is even worse - you need
at least twice the resolution for scaling to look reasonable.
It's not that 2x is double the number of pixels from non-Retina displays, it's just that 2 is the first multiplier up from 1.
In other words, the next logical multiplier to keep a 4.7" or 5.07" display's pixels small enough to still be considered scientifically in the "Retina" size range would be 3x.
Meaning today's 1136 x 640 screen goes to 1704 x 960.
I have long said that Apple should have gone straight to 3x with Retina Displays, as those are about the point where you start to hit diminishing returns from further resolution increases.
However the problem is that current apps would be scaled up 1.5x which looks very bad--and you would not want to scale up non-retina resources 3x instead of scaling retina apps 1.5x, because that would look even worse.
While legacy apps
already look rather bad today, current "2x" retina apps would look even worse than those on a device which used a "3x" retina display.
Seriously? Are you using a device with Retina or a regular screen like the 1st gen iPad Mini or iPad 2? Maybe it's just psychological on your part that you're seeing the pixels. I increased the font size to the max on my iPhone 5 in iBooks and couldn't see any pixels. I decreased it back to the size I usually read my books at and still no pixels. Looking at the photos I've taken, I will say that I take bad shots, but still I can't see the pixels at normal viewing range.
You're misunderstanding the problem. Text is always rendered at the native resolution of the screen when you do this. Increasing the size of the text will
improve its appearance.
Try making the text smaller and you should notice how there is not enough resolution to do a high quality rendering. It is particularly obvious when browsing web pages that use Asian characters.