When I'm shooting Safari in particular where you need this particular range 200-560, this Lens saves me having to carry 4 current Lenses 200f/2, 300f/2.8, 400f/2.8 & 600f/4
Well, I've never been on safari, so I'll certainly defer to your experience. But I personally don't see using all four of the lenses you list...I'd go with just the 200 and the 400 and the 1.4x TC. I'd have two bodies regardless; might as well have a lens attached to each. And I'd like to think that either I or my guide would have enough experience (or time) to know which to have ready. And to get into an optimal position for whichever lens was best. I also don't think I'd use the TC very much, except in slow-paced, deliberative settings...there's an advantage to using a TC, yes, but not a huge one.
So, that again brings us to the matter of whether the 200 and the 400 plus a 1.4x TC beats a slower all-in-one design. For convenience, maybe...but I'd still want the redundancy, which kinda does away with the convenience....
Which I guess is part of why Canon will make the Lens, some will use it, some will find it less than useful, but I'm reasonably sure getting this Lens in the first 6 months of release will be next to impossible.
When shooting something like Cheetahs or Leopards, you don't often have the time to consider attaching a Converter, plus on Safari there's the very real issue of dust getting into the Camera anytime you change a Lens, so the built in Converter would be a plus for me, on a Range of lenses to take with you ?? I travel from Singapore to Africa/Antarctica, when I get there I want to ensure I have as many scenarios covered as possible, better to have the Lens & not need it, than need it & not have it, Singapore's a long way from Botswana or South Georgia & excess luggage costs are not a factor when you travel with Long Whites.
I always shoot with at least 3 Bodies on three different lens set ups, 1Dx + 300f/2.8, 1Dx + 400f/2.8, 5DMK3 + 70-200 or 600f/4.