Probably at least another 23 years.
At a minimum, any new 35mm format mount would be backwards compatible with EF. For example, with the direction DSLRs have been going, I could envision the eventual need for fancy video targeted lenses that may need additional contacts for continuous AF/iris/aperture or who knows what else. But as long as the same image circle is cast, and the film plane is at the same distance, why come up with a whole new mount?