Well, film doesn't have a fixed resolution, so you can scan it at whatever pixel count you want, but there's a point of diminishing returns. For 35mm motion picture film, that's generally considered to be between 3-4K. Kodak claims 6K for newer stocks...
A digital intermediate can be thought of as what an original negative used to be - it's the final form of the film. Christopher Nolan aside, no one makes a movie entirely on film from start to finish anymore. Even film fanatics like Tarantino shoot on film, but complete post production entirely...
You also have to keep in mind that a 2K digital theatrical presentation is sharper than a projected 35mm release print. Films shot and completed on 35mm film are generally thought to have somewhere between 3-4K of real image detail - Kodak argues for 6K, but that's only for the newest stocks and...
The finished film only exists as a digital intermediate. There may be a negative, but it would be a film out of the 2K DI. In order to create a new 4K master, the entire post production process would need to be redone.