However, if recent history is of any value, I think that Canon will do an incremental upgrade - i.e. solving the problems of the current version more than reinventing it. There's a 400/5.6 prime too, and they probably wouldn't cannibalize it.
So sharpness-wise I don't think the new model is going to be a lot better. I think they will keep the best selling points being new IS and smoother bokeh. Maybe weather sealing? The push-pull design is a specific feature of this lens and they might well want to keep it.
Hey, thanks for my morning laugh – it was a good one!! How could a brand-new lens with the latest image stabilization system and the same focal length and max aperture NOT cannibalize sales of a 20 year old lens?? The only possible answer is by the 20-year-old lens being substantially cheaper, and no matter what, the old prime is going to be substantially cheaper. Besides, there are four 70-200 zooms and a 70-300 zoom in the L-series, along with the 100-400, and you think Canon is worried about cannibalization? For most people, a telezoom is the second Glenn's purchased after a standard zoom - Canon is very wisely offering a great selection for that choice.
As for sharpness, don't worry, the new lens will be significantly sharper than the one it replaces. Think 70-200/2.8 IS differential. I suspect the only people who believe there won't be a substantial boost in sharpness are Nikon fanboys (in or out the closet) who want Nikon to finally have an xx-400mm zoom with IQ that rivals Canon, which they do...and will continue to for however long it takes Canon to get the new 100-400 to market.