Here's a hallway at 21st Century Fox where I work (oh hey, now I work for Disney)
Rather than repurpose old iMacs, they go to their graves here. Far as I know, they don't even resell them, they just let them rot, unless there's a need for one somewhere. (Along with whole bins full of Mac peripherals.) Upgrading them is a PITA not worth bothering with at a large company level. It's the problem I've always had with iMacs.
You won't find a MacPro though- those are like gold. (Not the trashcans, they skipped even buying but a small handfull of those, but the actual tower Mac Pros. Those are all still in use in some form or other. I have one in my office doing server duties.)
Much as I never liked iMacs for professional work, I have to say, ever since I've had a fully spec'd i7 5k Retina iMac at my desk, I've loved it! It handles everything I throw at it for editing and the display.... I never want to look at a lesser display all day again.
Soon these same halls will be littered with a few newer gen iMacs as a few iMac Pros come in. No, I won't be getting one- don't really need one, but it would be nice!
I probably will get a newer modular Mac Pro when those are available next year though.
Never thought I'd enjoy using an iMac at work, but no complaints with the one I use every day, it's a workhorse. I can only imagine how awesome the new Pro is.