There's still major stations broadcasting at upscaled 720P, and considering how TVs have gotten massive and a lot more affordable, simultaneously a vast majority have migrated to portable devices that one can't even perceive HD on. Yes Pixel density counts but even 720P I think is overkill for handheld devices. But that doesn't stop the world from wanting 4K, and speaking of broadcast, Japan's NHK is working on 8K cameras!
Though more resolution really begins to make sense when it comes to editing, higher bit rates and better color spaces all help with grading and creating an image you like, but for cropping or any fine edits the higher the resolution the better indeed. Then again, did more MPs in our photographic cameras mean much? At that point where it became more than enough (when we broke 10MP?) it's not like anyone's pictures got that much better, yeah there's more detail and you can print a lot larger, even save a too-far shot by cropping more and more.
For now it's a big boys game where full out production companies can show off their muscle of 4K against the poor indie film shooters who can't go past 1080P when on a budget, like when 3D was all the rage (haha what on earth happened to all that?) it was meant to be a feature that only Hollywood & friends could touch so would protect them, and the marketing gets them back enough anyways, like all the views an incredibly boring video gets just because it has a 4K tag