I agree with a lot of the sentiment here. I can't bring myself to use crop after buying my 5d. So my 60d is just gathering dust. So I'm not convinced i like the idea of buying a gen zoom for that.
I don't like compact system cams so that doesn't sound like a good idea for me.
I think I agree with the comments re I'd be disappointed with an f4 lens.
Think that leaves me with canon 24 70ii or just go with a few primes and keep my rx100 handy to fill in the edges where neccesary. Light and simple sounds good
The truth is my photography is my hobby and I get a lot of relaxation from it. I can't sit and read on a beach like a normal person :-p
Your 5DIII and the 24-70II are like bread and butter - they just go together... You can carry one lens and get a lot of great shots... That being said, I would drop that shorty-forty in my pocket - I love what it does on my 5DIII...
+1 with Old Shooter