The previous incarnation of this lens was f/1.8. Yes, f/2 is still wicked fast for a 200mm lens, but it sure would have been nice to have seen it stay at f/1.8 -- or even go the extra half-stop the other direction to f/1.4! Sure, it'd make a big-and-heavy-and-expensive lens bigger and heavier and more expensive...but I bet most people willing to put up with the size, weight, and cost of this lens would gladly give up a bit more size, weight, and money for that extra stop.
The size weight and money expenditure of going to a 200mm 1.4 would be more than just a "bit." (Also, going from f/1.8 to 1.4 is 2/3 stop, not 1/2.) I presume the front element on such a lens would be roughly the same diameter as a 400mm f/2.8, and I don't even want to speculate on the weight. Way too much bulk for 200mm reach, in my view.
I know others, especially portrait folks, like to take shallow depth of field to the extreme, but for me, apertures larger than f/2.8 aren't really for actually shooting photos, but for easier manual focus. Sure, I'd gladly use a 200mm f/2.0, as it would make my hand and eye a manual focusing machine, but I would only actually use f/2.0 in extreme low-light circumstances. (I owned a Nikkor 200mm f/2.0 back in my Nikon days in the '80's -- great for night football on a dingy high school field with Tri-X film pushed to 3200.)
It's not just the 200 that's going backwards. The top-of-the-line 50 today is only f/1.2. It used to be f/1.0, and Canon even made an f/0.95. I wouldn't at all be surprised if the next L 50 is f/1.4.
The f/0.95, I believe, was an old rangefinder camera lens, and no such lens was ever in the EF lineup, or even in the FD lens range as far as I know. The EF 50mm f/1.0 seems most likely to have been a "look what we can do!" kind of lens for Canon, more for their P.R. than for any practical use by working photographers. I'm sure I have NEVER seen one of those in the wild, only at at the odd trade show or convention. I feel the same way about the 85mm f/1.2 (which has much the same design as the 50mm f/1.0). Extreme poor handling on the 85mm f/1.2 makes is a lens I'd NEVER buy, but I have rented on occasion and do see them in use from time to time.
I'd be hard pressed to call going from f/1.8 to f/2.0 or from f/1.0 to f/1.2 a "step backwards."
Who's up for a 12-pound 400mm f/1.8?
Ummm. . . No thanks.