I would not see a new Mac Pro Refresh until Early 2016 or at least when the 5K Display comes out.
5K is not a panacea. 4K monitors work pretty well. The newer single stream ones will probably work better over a broader set of offerings for the Mac Pro.
There is no good reason to hang up a Mac Pro release simply based upon the external monitor. There is little to no indication right now that Apple is even going to do anything to get back into the monitor business. Effectively Apple is in the docking station (with embedded monitor) game.
The Mac Pro 2013 released with Apple acclaimed 4K capability and no Apple 4K monitor. It would not be any shift in strategy at all when it comes time to dog and pony show 5K monitor ability to rely on 3rd parties.
Apple right now is focus on Consumer Products (iPhone,iPads,etc..), there are not to focus on the Pro Market because the Pro market does not move units.
In early 2014, Mac Pro demand outstripped Mac Pro supply/production. The Macs don't have to sell in the same unit numbers as iPhone/iPad. That is just misdirection. What is far more important is that people are buying them in sufficient (and preferably growing ) numbers. Overall Mac sales units are up, which very likely means Apple won't be dropping or backing off Mac R&D any time soon. Mac Pro's numbers are up but there was such a long drought prior that Apple will probably want to probe to see if that is longer term trend.
iPod sales are down and iPods are getting little to no R&D. Apple canceled the iPod Classic in part because "nobody" (so few) were buying it. That isn't particularly so few relative to iPhone... that was so few compared to how many they used to sell. ( folks may have swapped purchases of iPod for iPhone but again that is just following the market changes ).
Also when you buy a Mac Pro, you might keep it for 5-8 Years, Apple has to wait a while for new money, Its not like the iPhone/iPad which moves a lot of units/ has high markups and consumers usually upgrade every 1-3 Years.
It isn't so much that it is 5-8 years as much as the demographic change from folks moving from 3-6 upgrade cycles to 5-8 ones. Folks who don't need performance are an odd mismatch to the Mac Pro.
As large a set of folks coming out of a 5-8 cycles as 3-6 years ones is the same arrival rate of new purchases. Filling the pipelines is already done. It is the shifting of pipeline lengths over time that will stall demand for new boxes.
Shrinking demand is what will heavily influence Apples focus over time.