The way Apple works with its retina display is it's always 4x the resolution of the non-retina display. 2880p is 4x the resolution of 1440p. That's why it HAS to be 4x the resolution to maintain the real estate and size of everything - like we had in the 2013, 2012, 2011, 2010, 2009 1440p iMacs etc. Except now, of course, it's 4x as sharp as those old Macs, while maintaining the exact same work space. This is the way Apple has designed it, so we work with that - whether you think it's a limitation or otherwise.

Going from a 27" 2012 1440p iMac to a 2014 5K iMac is simple because all my windows stay in exactly the same place as they do on the 2012 iMac, except everything on the screen is 4x as sharp!
1080p quad-pumped to 2160p on a 27" display is NOT enough real estate for many of us in OS X, and for me there's no possible way I'd ever want a Retina 4K 27" display. It would be a MASSIVE downgrade from a 5K 27" display in terms of usable real estate, and in some ways a downgrade from a 1440p 27" monitor (due to that real-estate problem).