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If the 200-400L weight is not reduced for the production units, then that will IMO become a significant drawback of that lens model. Zoom or not, why get a 400 f/4 lens which weighs as much, and is probably priced as highly, as the 400 f/2.8 IS MkII prime ? A built in 1.4xTC is a neat "novelty", but I can very quickly mount 1.4xTC or 2xTC on 400 f/2.8 IS MkII and thereby obtain a couple of very nicely performing options, i.e., 560mm f/4 and 800mm f/5.6 combos.
Because it's a zoom and it will still be smaller (in diameter) than the f/2.8 prime. Also, while you can quickly mount a TC, the built-in one will still be faster to toggle, and with it being internal, will be great for inclement weather use.