The Airbus A340-300 is a very safe and reliable aircraft .It has beenin active service for nearly 17 yrs. The Airbus A340-300 is manufactured for a 20 yr service life. Most of the 300 series aircraft are getting old.
Giving it new engines may improves fuel efficiency. But still it may not match the efficiency of a twin engined airliner like 777. Secondlly the cost of maintaing an old airliner is very high.
Airbus is coming with A350-900. An all new coposite airliner that is more economical and comfortable and two decades ahead of A340. Naturally all airlies would go for the new jetliner.
The Airbus A340 and Boing 777 are two great aircraft. They both will be in operation for many years. The future however belongs to the Airbus A350 and Boeing 787.