Well, the OP does have a point in that the 400/5.6 is the longest lens that Canon takes out to the f/5.6 slowest aperture that they make across the range (that is the slowest that will AF will all bodies).
I think a 500/5.6 with IS would make a lot of sense. This lens would weigh about 1.7 kg and could presumably cost as little as $2400. A lot of people would find it convenient, and there is no simple EF XXX + TC option that gets to this range. If the 400/4 wasn't DO, didn't cost $6600 and wasn't controversial as for IQ then this lens also would not be needed.
The 600/5.6 makes less sense, mainly because the 300/2.8 + 2X TC delivers the exact same range and aperture, and probably would weight and cost about the same.