As others have stated (BTW - I'm a Canon user), if you've got Canon lenses, stay with Canon, but I would get a Full Frame camera if you can - stays true to the actual focal lengths of the EF lenses you've got now.
I recently (few months ago), bought a 6D (Gen 1), for $1399, just before the Mark II models came out. Was a good move - FINALLY I'm running FF. Love it.
I Don’t miss it in my newer, more expensive cameras. Myself, I never understood why it was really there. I was always told it was (there like the other said) to prevent excess light from coming in through the viewfinder when you’re Eye wasn’t up to it. Personally, I thought that was part of what the mirror was for as you click the shutter, it would pull up out of the way and block any light from coming through the viewfinder onto the film exposure area. Who knows.