There is never such a thing as overkill. Sure, it will be costly, but if any one has the buying power to get such panels mass-produced and lower the costs right now, it's Apple. Back in the early 90s I thought 640x480 VGA was amazing. 800x600 super VGA was mind-blowing. It does not mean that it was enough.So you don't think that a 5120 x 2880 display in 2012 would be both costly and overkill?
There is really no point in upgrading the display this year, when you can't discern the pixels on the current 27" iMac when viewing from an appropriate distance.
Being able to see more usually equals higher productivity. It certainly does for me.
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The "classic" MBP is egregiously overpriced.
Find another argument.
Your counter argument is horrendously flawed. We're discussing Apple computers here and therefore we have to use the closest to the rMBP as a comparison point, which happens to be the nrMBP. Whether they are both overpriced is not the point. The point is to make a comparison between a computer with a non-retina screen and one with a retina screen that are as close as possible in all other aspects to draw conclusions on how much Apple is charging for the new display technology. Then we can extrapolate that result and apply it to another product, such as the iMac.
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I think the barrier to larger Retina Class Displays at this time is adoption of Thunderbolt. I predict that it will require a Thunderbolt connection to use them. So, waiting on peripheral adapter manufacturers to ramp up and get costs down.
Look for a Retina Air next. The Apple Tv could presumably be a 4k model, which would put off introduction for a year and a half or more. Though a 2nd generation introducing Cinema class resolution won't work as a sale's plan due to the lower turnover for Television purchases.
IF Apple can develop a seamless iPhone/iPad/iOS/iTV transition. With the roundabout effect and future integration of iOS interfacing the desktop/laptop cycle should refresh in 3.25 years, just in time for the "Next Big Thing".
Connecting an internal screen to a GPU in a computer such as an iMac or a laptop has nothing to do with Thunderbolt. Nothing whatsoever.