I've been thinking about this for 2 years now. I bought my first Fujifilm camera (X-T1) a little bit more than 2 years ago now, and while it's a great camera, for some things I still haven't trusted it a 100%, and still don't since especially the autofocus is not good enough for some things.
So I've kept my Canon 5D MkIII and lenses as a "security net" but I've hardly used it and now after having my third Fujifilm camera, the X-T2 which still is far from perfect, but still a lot better when it comes to autofocus for example, I think it might be time to let it go.
The last two years I've moved away from shooting landscapes (even though I've started to think about coming back to it again), and I first started to shoot a lot of street photography and now mostly portraits/models. Sure, the 5D MkIII being a full frame body can create even shallower DOF than the X-T1/X-T2, but I don't really care that much about the difference since I really didn't shoot that much portraits before I started to use Fujifilm, and quite frankly, many times even focus on the Fujinon XF 56mm f/1.2 becomes hard to hit exactly right since the DOF is so shallow, and I don't that that much shallower than that is not really any point of having.
And when it comes to shooting landscapes, to be honest, the Canon falls short compared to Fujifilm since Fujifilm has a much better dynamic range, something that is very useful and important for landscape photography. Sure, there are ND filters to help balancing exposures and there is of course also a limit for what Fujifilm cameras are capable of, but there's still quite a difference. I'm still gonna buy quite a few Lee Filters Sev5n filters for my Fujifilm setup.
And I've also thought about things and, I don't really need a super fast autofocus, since I don't shoot sports or wildlife, I just need one that is accurate for portraits and landscapes and like I said, the Fujifilm X-T2 is quite a bit better than the X-T1 and probably not that far behind the 5D MkIII, so it'll be okay for street photography.