How could you possibly be drawing this conclusion? One of the biggest points of separation between devices, especially with larger screes are the ability to reach into the high-resolution realm.
Prohibitive cost of big high-res panels and not fast enough GPUs. This are my reasons. For something like Air this could work, as the panels won't be that expensive.
You might want to check out
this article, and see that iMacs are almost retina quality already, and this 5120 x 2880 is not the correct resolution for true Retina Display graphics. You can't just double the current resolution. There actually is a formula to determine how many pixels are needed in conjunction with the screen size so that they are not discernible to the naked eye at the correct viewing distance.
It actually wouldn't take much to make most of these models Retina Displays.
There is one essential thing that you are missing here. If you render text or draw images on a 160ppi screen suing the same resolution (say, around 10x10 pixels per character), you will have to stick your nose into the display to actually see anything. You tus have to adjust the size of elements to the PPI, otherwise, the high-res display will be totally uselles. Look how it works with the iPad - basically, they render everything bigger than with the smaller resolution, than it appears the same size. Read Apple's developer pages on resolution independence, they describe it very neatly.
To support this, Apple has something called the scale factor in their API. This is simply a number which controls the look of things liek font and UI elements. Per default, this factor is 1. Say, you have a 100x100 pixels window in your application, then it will be displayed as 100x100 on the default screen. For a high-PPI screen, this windows would be very small. Thus, Apple sets the scale factor to be bigger with such screens. The API will then convert the dimensions automatically, e.g. resulting in 150x150 window on a display with 150% as much PPI.
Now, the trick is than Apple seems to support only x2 scale factors in its developer tools. They have these things called HiDPI modes - which basically emulate 2x2 increase of resolution on the same screen size. Based on this, I find it unlikely that they will implement any other resolutions. If they go this way, 3840 x 2160 is only 1080p quadrupled, which means that 2x scale factor will make it a logical 1080p display. This would be a step back indeed, as it will actually have less useful screen estate than the current 1440p screen. To have the same screen estate, you need the scale factor of 1.5, this would give you logical 1440p mode. Now, I am unsure whether this won't introduce artifacts in some cases (because you'd have 1.5 pixels for each logical pixels) and also Apple has no single HiDPI mode which would let you tests these things out.
Hence, I do not believe that retina is coming to iMac yet.