Adobe has a release candidate that works, see
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A 200-500 is not a replacement for the 100-400 or 400 prime.
Also while the use of extenders has always been questioned, in this case it is built into the lens. Canon can fully optimise the performance of the built in extender and the rest of the lens characteristics beyond what an external generic one might offer.
I have no doubt the 200-400 will be a great seller for its class, even if not reaching huge numbers from the likely very high price. Lots of people would love the zoom range it offers over primes in a similar range.