It's not just about higher-resolution displays, it's about having more visual information at lower resolutions as well. A 12K picture properly compressed down to 1080p will look way better than a 1080p picture at 1080p (given good equipment for both capture systems). It's not only sharper, but you also get extra benefits like being able to crop the picture (which can be used not only to crop, but also for software stabilization, Ken Burns-style pans & zooms, etc.) & future-proofing of your footage as well.
We're still a ways away from digital being equal in quality to actual film, too. I was watching the Ten Commandments the other day on my 4K TV & the picture quality was still pretty dope, despite it being filmed over 60 years ago. Same with stuff like Gone With the Wind & various other old movies shot on good-quality film with good digital conversions.