I think the 100-400 and 200-400 appeal to totally different markets, a bit like Bentley and Audi, with Canon being V.A.G.
200-400 is the Bentley. Expensive, best of best, rocketship perfomance and weighs a lot.
100-400 is the Audi. Many of the same parts, cheaper, performance is great, but not rocketship and half the weight.
I can't afford a Bentley (or 200-400), but could stretch to an Audi (or 100-400 v2). Either way, I like to drive fast, and there is always a "fast enough".
I can live without the the walnut dashboard, extra 1 second 0-60 and 200mph+ top speed. I can live without a constant f4 and with slightly less IQ, just to be able to own something that comes close(ish), goes up to f5.6 and out performs a 3rd party lens.
- 200-400 is for professionals and rich amateurs.
- 100-400 is for professionals with a weight limit and keen (and not so cash-flush) amatuers.
I can see both lenses selling and complementing each other well.