Years ago when I worked at EK, they had outstanding orders for both the 340-600 and the 777-300ER. I asked someone in the know why we had orders for 2 types that essentially had the same mission and was told that if oil stayed "cheap", the 340 was the more economical airplane over 10 years. It burned "a lot more fuel" and had higher maintenance costs but its cheaper acquisition cost made up the difference. If oil got expensive, the 777 ended up being more economical over 10 years. Oil got expensive, the 340's were cancelled and the rest is history.
I would expect that with the rock bottom residual values of the 340-600, that it might not be a bad buy. The big questions would be how long oil stays cheap and how much more are maintenance costs going to be with the older machines.