The eventual "modular Mac Pro" is going to serve the crowd who has the exact need as yours. As late as the Mac Pro may come, the current iMac and then the December iMac Pro are both not catering to the same crowd. The iMac still serves the home to semi-pro user-cases where the AIO nature is desirable or not detrimental to their workflow, and as a machine the top end BTO iMac has very respectable specs on top of it. Apple's idea seemed to dig further on this front, as indicated by the words during the roundtable and then again this WWDC, they observed a trend of "some" pros going all the way iMac (from cMP or tcMP), thus the idea of an even higher spec'd iMac Pro emerged. For this crowd, according to Apple, having a screen permanently attached to it is what they want.
Having a 27" 5K monitor as a must of course has its pros and cons, but for most use-cases it is not an issue. In the past, where even the Cinema / Thunderbolt Display still didn't offer anything higher than sRGB and not being able to be hardware calibrated etc meant a professional external solution was always needed, but nowadays with the iMac's DCI-P3 dithered 10 bit paper spec, pretty much only a small subset of print and video color grading tasks would find it short. And then if you must add your own, better or bigger display along with the iMac is easily served by the TB3 bandwidth provided, multiples of display in fact. If space is an absolute issue then you got the trash can Mac Pro for now and the modular Mac Pro next, which Apple has no issue admitting they have fuuked up the large void between these 2 releases and you just gotta stuff it up for now.
I think the bigger problem lies at the lower end of the Mac line up really, now if you find the Mac Mini 2014 underwhelming in specs, your best bet is to get a no touch bar MBP and run it headless, but it is not very cost effective as a desktop replacement as the imaginary headless "Mac X".