It doesn't matter if you have the best there is. Someone will still want "better".
Specifically on a hypothetical 100-400L II, I think the most significant changes will be in areas other than the basic optics. The push-pull vs. twist argument will probably go on forever but at the end of the day you get what you're given or look elsewhere. Updated IS would be a given, and I'd expect a weather sealing upgrade too. If the optics are improved on top of that, I don't think there's much to complain about other than the price tag.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye