Marco Arment explains why there’s no Thunderbolt displays on the horizon.
Pushing this many pixels requires more bandwidth than DisplayPort 1.2 offers, which is what Thunderbolt 2 ports use for outputting video signals. (I wrote about this a few times.) Doing it right will require waiting until DisplayPort 1.3 in Thunderbolt 3 on Broadwell’s successor, Skylake, which isn’t supposed to come out for at least another year — and Intel is even worse at estimating ship dates than I am, so it’s likely to be longer.
It may be possible to use two Thunderbolt 2 cables to power a 5K display, but only if the GPU could treat each port as its own full-bandwidth DisplayPort 1.2 channel, the sum of which represented one logical display, and had the panel using something like MST to combine the two at the other end. But the only Mac with more than one Thunderbolt bus (not port) is the current Mac Pro, and I can’t see today’s Apple shipping an external display that none of their laptops can use.
There is no way Apple will launch proper 5K retina Thunderbolt displays, on par with the new retina iMac, anytime soon. I based my predictions on the matter on the same basis.