Again, I urge you not to think of this as an engineering problem, that's the wrong approach; pricing is a business problem, not an engineering problem.
Even if all the video crap in DSLSr would really be "for free" (which it is not) I still would like to have a choice. And no, I am not interested in agrossly overpriced, useless UI retro Nikon DSLR (Df) but much more so in a a 5D IVs ... "video-free" in a Sony A7 sized body. :-)
I'm trying to think of anything that goes into making a good video DSLR that makes it either more expensive or inferior as a stills camera compared to a stills only model.
From a hardware point of view, things are different. It's bought us sensors which can run for longer before they overheat; this is good for long exposures too. It's come hand in hand with live view; this is great for landscape and macro. It's bought us live view AF and flip out screens; ok, not much use for me in stills, but grabbing overhead shots are now more likely to work out. It's bought some manufacturers (and Canon with ML) peaking; great again for landscape and macro. And it's also bought us in some cases (5D3, but not 6D, 1D X or 70D) sensors which offer perfect down sampling to video resolutions; for photography, the 5D3's sensor is a big step up over the 5D2, even if some people are jealous of the D800's resolution or it's DR. It's bought us STM lenses; no match for USM, but a step up over the buzzy micro motors. A hybrid EVF could assist with photography if manual focusing, and for checking exposure/DR.
Software costs money to develop. But once developed, it's essentially free to manufacture. So while including video increases the development costs of the software, the additional sales it generates potentially lowers the cost of the camera.
Additional video tax is avoided by crippling the video recording time to under 30 minutes.
So are photographers really held back by video enabled DSLR's?
edit: It's also helping drive forward the data throughput of sensors, camera processing and storage; all of this development done partly by Canon and partly by other companies (ARM, SanDisk etc) gives more headroom for the MP/FPS compromise on top end stills cameras.