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A 4K 21.5" will be a sweet machine but it's not out yet because there's not much of a market for a 21.5" iMac north of $1,799...which would be the likely price.
 
A 4K 21.5" will be a sweet machine but it's not out yet because there's not much of a market for a 21.5" iMac north of $1,799...which would be the likely price.

Based on my experience with my job (video editing with a marketing department), I don't find that very true. I would have grabbed that machine in a heartbeat over the 27 inch one, but since they didn't release it I went with the 27.
 
I think they could do a 23-24" 4K/UHD retina imac for around $2k and they would be able to release a matching cinema display as it will be 4k so displayport 1.2 could handle the bandwidth. They could EOL the non-retina 27" as this would replace it in the pricepoint.
 
The margins are probably higher on the 27" and with the novelty factor, they would want all the early adaptors to pay top dollar. Once those people have gone through, Apple will roll out Retina to other models and also drop prices, just like they did on the rMBP's in February 2013. Early adaptors get hosed, many times while they are giddy with excitement for it.
 
There might be noone making appropriate panels (3840x2160@21.5" - which would have lower dpi than the 5k 27", just like the 21.5" imac has lower dpi setting than the standard 27"). Even with the same dpi as the 5k 27", cutting the imac 5k panel down in size would result in...a 21.5", wider 4096x2160 imac, or a 20.5" 3840x2160 imac, both requires a change in form from the current 21.5".
 
As said above, the 13"rMBP took months before it was released. Maybe some constraints with the retina panel's production.

Also it's much much smaller, which means less space to cram stuff into it, so it's probably more difficult to engineer.
 
It's too small. I don't see Apple keeping the 21.5-inch models around for that much longer.

It's not small, the 27 inch is just huge

I'm expecting an iMac with retina 4K display (3840x2160) and that and the 5K one to replace the non-retina ones and their price points. That said, Apple really should put all their Macs at a lower range price. Here's the lineup I want:

iPad Pro (see my thread): March - start at $699
MacBook Ultra (a thread will come for this): October - start at $999
iMacs with Retina Display (see above): October - $1099 and $1799
Mac Pro (small updates): October - $1999 and $2499
 
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