I only ever see the 400mm f/5.6Ls stuck to a massive tripod with hood fully extended and a guy in a baseball cap glued to the eyepiece.
You must not go where there are a lot of flying birds. The 400/5.6L is commonly used to capture birds in flight - long focal length, fast AF, easy to handhold, reasonable cost, and flying birds demand shutter speeds that render IS pretty ineffective. Still, I'm firmly in the camp of wanting an IS version of this lens - then, it would be useful for stationary subjects, too.
When I was back home in Texas, the only people I saw near flying birds had shotguns in their hands, not cameras.
Here in Australia, I find that the lenses of choice are typically much smaller, as the birds are quite tame and the white and black cockatoos and other parrots will let you get pretty close, and anything under 300mm is suitable for bird watching near the beach. Usually I find it's just a long waiting game for some kid to run up and spook them into flying away if you want to catch them in flight. Even in the bush, I believe you'd be hard pressed to need something with a longer range. Even the galas will let you get within 5-20 feet, and give you a massive show of pink and white feathers up close and personal.
Once, I managed to get close enough to a great blue heron to fill the frame (a 1.6x crop frame, mind you) at 400mm. That was after about 20 minutes of careful approach.
EOS 7D, EF 100-400mm f/4.5-5.6L
IS USM @ 400mm, 1/640 s, f/5.6, ISO 640
...but the majority of the time, even 400mm on a crop body is not long enough.
I think the 70-300 L and the 200-400 L will replace the 300mm f4 IS, and the 100-400mm.
The 70-300mm is quite possibly the replacement for the aging 100-400mm. A zoom cannot really replace a prime, though, RE the 200-400 being a replacement for anything in the sub-$2K range, that's like saying the 1DsIV is the replacement for the 7D - preposterous.