You know what I mean! A new version with exceptional sharpness (just like 200-400's sharpness in comparison to 400 f/2.8's as Roger Cicala mentioned) will affect sales of 200-400. True some determined professionals will prefer the f/4 but many may choose the new 100-400 if portability is of paramount importance (we all grow older). On the other hand I guess the new 100-400 would sell extremely well!!!
So, the one that is twice as fast and heavier with a more tank-like build won't sell as well, except to dedicated professionals - so why should Canon make the cheaper one that would sell extremely well? Remind me...why is there a 5DIII? Because my initial statement could just as well describe the 1D X.
There's a place for both...
I often hear the statement that cannon will not come out with a lower or mid range version of a product because it will cut into sales of the top of the line unit. I couldn't disagree more.... it's the millions of lower end units that keeps the lights on, not the hundreds of top-of-the-line units. If the logic were true, then Canon would only make 1Dx's and $10,000 primes.... get rid of the p/s cameras because they cut into 1DX sales....
Canon will sell hundreds of times more 100-400 zooms than the 200-400 lens. In the mass market, very few will pay $1500 for a lens and a miniscule percentage will pay $10,000. The 100-400 sells so well because it is affordable, works better than the kit lenses, and gives a longer reach. Plus, people love zooms.... zooms outsell primes....
Rework the 100-400 and the 400F5.6 to the latest technologies, give them both IS and the same level of build. The 400F5.6 will be sharper, will cost less, and be outsold ten to one by the 100-400 zoom because PEOPLE LIKE ZOOMS!