It has been said before, but I like to give my 2 cents on this too.
-There are physical limitations for the optics that make it impossible to make them smaller, without loosing some aspects of fotographie (DoF) and qualitiy (diffraction).
It does not make sense to build a small camera, and attach a big lens to it (ever seen a NEX with a 24-70 f2.8 Zeiss).
-The main problem with small cameras is, that there is not enough room for all the buttons needed for fast operation.
So ergonomics is another point why the formfactor of DSLR won't change that much. You can compare it to a laptop computer, it would be easy to make it much smaller with today electronics, but the display and the keyboard need a certain size.
I think that mirrorless (ILC/EVIL) will play a big role in the future of cameras. For many occasions they are a nice tool, small with great IQ. I also think that there will be bigger mirrorless cameras in the future, that will be positioned like a DSLR. Actually, it doesn't matter if there is a mirror or not, important is the image quality and the ergonomics.