I am with tonyjuliano
on this one. A few generations of camera can mean night and day difference. The sensors are ever evolving to deliver better and better performance. My 7D gets the grains in low light when I even bump it to 3200 ISO with a fast lens (f/2.8). Granted, that is a lot of ISO, but the new 5Ds is boasting double that at 6400. I am excited to see what kind of images will come out of that sensor.
Also, don't forget the image size. 7D puts out 18 MP images. More than enough for anyone really. The 5DS will spit out over 50 MP images. That is a lot of detail. The new sensor will be crispy for sure. That is an exponential evolutionary jump in sensor and the 7D and the 5DS are closer generationally than the 5D MkI and the D7100. The D7100 is a great camera and has a lot of built in "so-it-all-ness" that general consumers want. The 5D lineup is a professional level body. It all depends on what you are going to use them for.
To mirror what tony said earlier... I could slap $20 grand worth of camera and lens in a person's hand, and that does not mean that they will produce anything worth looking at. It is what is in the 8 inches behind the viewfinder that counts. If you know your way around a hobbyist camera and can crank that thing and know it's limitations, you will be able to produce images that move people.
I have much respect for film guys. They had to get it right, or very close. Every time they pulled the trigger it cost them money. Us, not so much.