It's funny, last winter everyone was saying "there's no way the 200-400 is coming out this year, don't tell anyone to consider it in their purchases." And here we are, the lens released this summer, and people have it in their hands.
I think there is a sort of "critical mass" of rumors where you can pretty safely bet on something happening.
The mass testing of the 200-400 at the Olympics was a pretty good indicator, and there were multiple in hand prototype rumors.
When someone says "it's coming just trust me" that's one thing. Once they start showing the thing off at trade shows, that's another.