...
So, Apple MAY release a 5K thunderbolt display,
Thunderbolt is highly unlikely. Thunderbolt doesn't solve the problem. Thunderbolt v2.0 only carries DisplayPort 1.2. DP 1.2 needs two separate streams on individual cables to do 5K.
Even TB v3 is still only a carrier of DP v1.2. Still isn't going to solve the problem.
Routing two TB daisy chains into one device is likely going to surface some Rube Goldberg "solution" even if did want to shovel all the DP v1.2 into two (or more ) Thunderbolt cables. If the chains are of unequal length I suspect it doesn't work either (timing for the two halves probably wouldn't arrive in sync).
Apple could make a two input DisplayPort monitor like the Dell and HP models, but it would not be a Thunderbolt Display.
More than a bit farther down stream there will be a TB v3+ controller that carries DP v1.3 (or better), but that will be hugely decoupled from the current Mac line up ( including the MP 2013 model ). Eventually the technology will evolve so there is a straightforward solution. That technology isn't in the current line up.
but provided there's a driver update for the nMP, it would ONLY work with the nMP, and only with a dual thunderbolt/mini display port input.
There are other Macs with dual DisplayPort outputs. iMac and Mini 2014. I don't expect Apple to go for this stop gap solution though. I suspect they will kick the can down the road until DP v1.3 comes and only do external support for that. Primarily because it can be more elegantly be done with just one cable [ custom two cable bundle inside of 5K iMac aside I don't think Apple is going to want to deal with ( support ) two cables. ]
It isn't like they have added driver support for the max number of single cable MST DP v1.2 monitors out there now. Some yes, but not all.
From the Dell 5K side though.... they just need the two DP v1.2 inputs and DisplayID support.
http://en.community.dell.com/support-forums/peripherals/f/3529/t/19609535
[ For now it on Macs it is only 4K support on the 5K monitor. But currently most TB v2 Macs technically have two DP v1.2 capable outputs. ]
As the other aspect of the thread discussion outlines Apple has other performance fires to put out rather than technological stopgaps that only an extremely narrow set of monitors need.