It sucks that you had to sell it. How was it to use? Since you've had both, how would you compare it to the 100-400Lii? I know the obvious: the 100-400Lii is lighter.... and slower. So, just how much better is a $11k lens than a $2K one?
It's a different category really. The 200-400 is really a ¬200-600/4-5.6, so that's how I think of it - as a versatile super-tele. IQ is close, AF is close, but that extra reach means its a Super and not just a tele-zoom.
The 100-400 is really cool as a walk-around, and with today's ISO performance it becomes less relevant to use a 70-200/2.8 unless you need/want that f/2.8 look.