Until someone comes up with a better lens, Canon has no real incentive to improve a already good lens.
The 100-400L is a so-so lens. It's capable of great shots, but it has problems. Wide open, its sharpness varies with IS element position much more than it should. IS performance is terrible (I mean, really, really terrible). Handling is lousy because the natural place to put your hand is on the lock ring or AF ring. The AF ring should be farther out and not turned by turning the lock ring so you aren't accidentally manually focusing when you don't intend to be.
We're getting f/8 AF back, and we're getting higher pixel counts. We need a 100-400 that's designed with the 1.4x TC III in mind, that has vastly better IS and better handling, and that stays sharp wide open regardless of where the IS elements happen to be.
My 70-200/2.8L IS II with 2x TC III performs as well at f/5.6 and 400mm as does my 100-400L when the IS on the 100-400 is turned off. With the IS on, the 70-200 is more consistently sharp and has vastly better IS and handling.