The only Canon camera you can downgrade to that wouldn't result in lower IQ would be a 6D. You would be giving up a few very nice features, gain a couple (GPS & WiFi), and reduce size.
As for lenses, the photo you just posted I assume was at f/2.8. If you get a 24-70/f4.0 you'll probably give up just a little bit of IQ (from the test I've seen so far, the 24-70/2.8II is the sharpest in it's class), but will get the ability to handhold in a couple of stops lower light with reasonable results.
All in all, stirckly speaking I don't think you can get better results over all by downgrading your equipment, but you can reduce the weight of the equipment you are carrying around and may be able to increase your handholding capabilities by up to 2-stops perhaps, but otherwise you'll be downgrading for the most part.