Upload & Sell: On
The reasons PetKal notes above for choosing the 400 DO vs. the 200-400 are similar to the reasons I sold my Sigma 120-300 2.8 OS to buy a 400 DO off the B & S boards a few months ago. Sure, the ability to zoom in/out, especially with a 2xTC attached, was great. However, the weight difference was obviously significant (especially as I typically hand-hold and pan at racetracks), and the size/length started to matter as adding a 2xTC to the Sigma creates quite a long lens compared to a 400 DO with a much smaller 1.4TC.
As far as people saying "who cares about the weight go to the gym!" I'll say that I'm 6 foot 3, 200 lbs., and run 4-5 miles most days of the week. So while swinging around the Sigma was physically doable, it didn't mean it wasn't a pain in the butt to do so at times, especially in the summer heat, nevermind the weight of my backpack with it, along with a shorter zoom of some sort in the bag.
Sure, I miss the zoom at times, but for what I use it for, the 400 DO works better for me. I see the 200-400 as the perfect lens for Safari shooters, where changing lenses in dusty environments isn't preferred, and the action could be a number of different focal lengths away with no warning or time to plan. Or, for those who use a monopod most of the time for what they shoot. I think for those people, this lens will be money well spent, and they'll use the 200-400 for 20 years and consider it an easy investment to make.
As far as using the Sigma 120-300 in place of the 200-400...to me that's still up in the air due to the failure rates of the first 120-300 OS. Because- assuming this zoom range replaces a shorter zoom and super-telephoto in your kit, and you traveled somewhere with the intention of using the lens, what happens if it fails? To me part of the money spent on the 200-400 is the assurance that the lens will continue to deliver no matter what, and you won't waste money on a trip where you ended up unable to shoot. Hopefully the new 120-300 OS "Sport" model changes that trend of the last one.