Retina support becoming mandatory…
Don’t think so within the next 1-2 years. Too soon for most Mac setups. 
Apple still selling HDs… Using Broadwell it will be updated in no time. I had lots of expectations in the new Imac Retina 21", but it’s lacking a decent graphics card, outdated chip, and selling SSD as something new.
They sell the package - and sure its a new form factor for a 4K
They’ve had a 21" iMac before
It’s new in that sense
All manufacturers still sell “old” tech in new form factors or at new price points
That’s business
Yet there’s no hint of when an external 27" Retina display will be available, not that my aging 2012 MBP can drive it anyway…
I am contemplating replacing my 2012 rMBP with a 2015 rMBP and a 4K TV (so I can also hook up the PS4). Anyone done this?
It could
Ohhh…
as far as I understand it it could drive ONE
Thats the exact vintage I have
Currently I drive 2 externals - not retina though
One a 24" (1920 x 1080) via displayport
The other a 27" (2560 x1440) via HDMI
Lots of screen space
My understanding is this is theoretically capable of driving both the internal retina AND one external retina - if such a beast existed.
lol… Only if older MacBooks got better with age 
I think at this point, a 4K TV is the thing. I really like the new LG wide wide wide screens, but being low resolution displays puts me off.