I wouldn't make it seem as though all of the major studios use 5k iMac's, seeing as 6-9 months ago Sony and Universal in CA both had Avid bays running on some Dells, as well as a few old gen Mac Pro's...
Granted, I've been to a few major studios that have had a few iMacs for quick editing; but that doesn't say much, as anyone in the professional media industry knows, you can even use a 7 year old 'Dell 7100' to edit a 1080p video for TV with no problem at all. And you can pick that up for $150-200.
Obviously, no one is using iMac's for intensive tasks, like grading and CG, etc. Those 5k's fall flat on their face due to the graphics limitations. But they are perfectly fine for editing video; as is just about any modern computer. Editing does not require a fancy setup. If you simply edit 4k video and nothing else, the iMac will be great for you. If you want a computer that can handle everything, obviously go the PC or old MacPro route.