It really bothers me that the 2013 Mac Pro might not support upcoming 5K Apple displays.
What upcoming Apple 5k displays? Until DisplayPort 1.3 / Thunderbolt 3 is readily available there is
no interface that will support an external 5k display over a single wire.
The 5k iMac works because Apple can use a non-standard internal connection.
The Dell 5k uses two DisplayPort 1.2 connections (basically, behaving like 2 screens) - so Mac Pro compatibility is a matter of OSX support, not hardware. Apple could go down that route with their own display - but it seems a bit kludgy and, on a MBP or iMac, would knobble all of your Thunderbolt capability.
More likely, they'll hold off until there's a credible 5k-capable interface available - which won't work on any current Mac. Or maybe they're just out of the display market.
Also note the 12" MBA rumours that show it with a new USB 3.1c port as its only interface. If that turns out to be true* then (a) Thunderbolt really will be dead and (b) any new Apple displays will probably have USB 3.1c connectors (which can carry DisplayPort).
I think people over-estimate the importance of Apple displays to the Mac Pro. The Apple Cinema LED/Thunderbolt Display has always been a "prosumer" product that makes more sense as a laptop peripheral than a "pro" monitor. Mac Pro customers are just as likely to get a real "pro" display from NEC, Iiiyama, Eizo et. al.
(*note the if - its still a rumour. Of course, it could still turn out to be a TB3 connector or, since these new connectors are all 'soft', TB3 could end up using the USB3.1c connector).