Thanks Falconeye for welcoming me here!
Anyone who has ever used a video camera, whether consumer or professional ENG/EFP, has used a "mirrorless" setup, focusing a lens on an electronic sensor, using an electronic viewfinder.
In other words, to me, "mirrorless DSLR" = video camera, for imaging purposes. I think marketers have to justify their jobs to promote that in DSLRs which is already achieved in video cameras.
Once all optical, analog elements, (namely viewfinder and film, but not lens of course) in a film camera are removed and replaced with it's electronic counterpart, what do you now have in your hands that we've had for a long time now? (a video camera). And it's ability to record single image files vs "movie" files is a function of what? (I say programming - you either program it in, or you don't).
How many years has it taken to convert still film/optical cameras to still/digital electronic? And during that time, the "mirrorless" electronic video camera has existed side-by-side. I'm sure it can be done, to make a camera that can shoot still images and video well enough to be a "do-it-all" tool. But would one want to do that, especially at the professional level? Don't get me wrong: I think it's nice to have video recording ability in my little still camera when I'm out and about on holiday, from a non-professional perspective. I already use it in that way with my point-and-shoot. But to the serious amateur videographers out there who produce and edit videos, I say, save some money and get a real video camera. If you want interchangeable lenses, save even more money. It will save you some headache in the end, and you will be more professional for it.
And to end my comment, I'll say it again: "Where is my FF Pentax??!?!"