From what I understand, 4K is about the right size for getting most of the detail out of the film. It's sort of 35mm's native resolution. But because it's an analog medium, it essentially has unlimited detail - doing it at a higher rez just means better detail on grain and so on, and also allows more latitude for future digital work.

In the same way, you could adequately document a painting with a 30-megapixel photo, but if you want to completely document it, you'll be working with x-rays, depth maps, restorers, color technicians etc. It depends a lot on your goal. For a great-looking copy of a movie you'll watch happily, 4K is great. But there may be other uses or applications out there that require or benefit from more.