I have both a 7D and 5Diii and prior to the 7D, it was a T2i. I really liked the 7D but made the switch to 5Diii based on all the reviews and excitment from the people here. I thought about selling my 7D since I probably will not use it much. There will be times on vacation that I will take it with me just in the event something happens to the 5Diii.
My opinion is that if you have it sitting around and don't sell it now, you will get less and less for it every day it sits. So sell it if you need the money, keep it if you don't or give it to someone you care about (my daughter now has mine). In no way do I think it is a bad camera - it was a great camera but the 5Diii is even better. I think the people that have the older cameras know they won't get much for it and it would be a hassle to sell. I still have a Sony PS2 somewhere in the house (I think).
Maybe you look at it as your go to-work-camera.
. I too hate that argument and see it usually posted as you did, insinuating that the old turns to crap. Of course it doesn't, but the newer model is better. I'm not trying to make a bifg deal of this, just bored at work but it is in my top three pet peeves with the others being "It's not the camera, it's the photographer" and "It felt so small in my Paul Bunyan sized hands". 
