The Mac Pro isn't going to have 5K capability until Thunderbolt 3. Thunderbolt 3 will come with Intels Skylake processor which won't be a Xeon processor until probably late 2016 or early 2017.
There is no way that Apple would standardize the iMacs around a Retina 5k display until after the Mac Pro has 5k capability. Anyway I see no hurry for Apple to take the iMac line to 5K resolution because any benefits for the average user are not going to out way the additional cost of the display. Those benefits right now are clearer text. The graphic capabilities of 5K are aimed at graphic professionals and prosumers.
Also Apple isn't going to reengineer the iMac Retina 5K a few months after releasing it. You don't simply drop a new generation processor onto the logic board. On the other hand the other iMacs are over do for a refresh. I don't see much of a gain with Broadwell but Apple will toss in a few other goodies to excite consumers.
Maybe. My view is that with Apple, like many premium brands, it's about the marketing; and taking someone over to the in-store desk saying, "So this is basically that one, but worse." isn't going to fly well. The MBP got killed off so quickly by the rMBP, I'd be surprised if the same didn't happen with the iMacs.
The cost difference is getting towards negligible (£240 at the cheapest, closest comparison), given the marketed benefits - 5K screen, faster processor, faster graphics. I'd be surprised if the average 27" iMac buyer didn't fork out for the retina now. It's £1,759 vs. £1,999.
Whilst I agree that the benefits are limited for your average computer user, the reason
I bought one wasn't for the retina - it was because my iMac needed replacing. That's the primary reason the 'average user' buys an iMac, or any new computer - their old PC/Mac is knackered. Then they're faced with £1,759 for an old iMac, or £1,999 for the all-new 5K iMac. Again, marketing kicks in. We all know how well Apple use prices to up-sell.
Updating the now-old iMac with a newer/better processor than the new iMac doesn't seem sensible either. You'd risk killing sales of the retina.
That's just my opinion. You might be right, I might be right. Ultimately, though, high-DPI desktop displays are going to continue to become more common. The iMac is one of those.