Also, where is that new Thunderbolt display?! (Just gonna keep commenting that till it's here)
Probably waiting for someone to build the 5120x2880 27" panel needed for a full HiDPI TBD replacement and invent an interface to drive it.
...or for 4k to evolve beyond what is obviously a horribly kludge of a interface that relies on the OS to 'support' particular brands of monitor.
...or for OS X to support a '4k' DPI setting - somewhere between regular and Retina/HiDPI that gave sensible-sized system fonts and icons on a ~30" UHD display.
...or for Thunderbolt 3 and enough bandwidth to drive a 4k display and the ethernet, USB3 etc. docking functionality and Thunderbolt-through that is the USP of the Thunderbolt Display.
...or for the ~21" UHD panel that would be sensible for a 'Retina iMac' running in HiDPI mode, and a matching ~21" TBD to fulfil the main purpose of the TBD: a premium-priced 'prosumer' accessory for the MacBook Pro.
Or maybe for the world to realise that the only people who need a 4k desktop display right now* are video pros working on better-than-1080p material and (like 1080p before it) its a godawful antigoldilocks format for general computing.
Then maybe they'll be able to produce a 1440p TBD with USB3 without getting shredded by the 4k wannabes.
(*I'm sure higher resolutions will come in time, but the more I read about 4k, the more obvious it is that its not yet fully baked).