As has been stated, the A310's will be fairly old by the end of the decade. Older aircraft become much more expensive to operate, and it seems as though a company with money is better off getting new aircraft that are more economical to operate. Air Transat fortunately is in a good enough position to be able to afford the acquisition of new aircraft from time to time.
It is more debatable whether or not it is wise to get rid of the A330's, but as has been suggested, perhaps they are not ideal from an operational standpoint. Their size can be a hinderance at times, plus they lag in reliability compared to some of the competition.
But again, I remind you all, this aircraft replacement will take place towards the end of the decade, so arguing based on the current situation is not realistic. You must look into the future, as Air Transat is doing, in order to come up with a sound decision.
"it's kind of like an Airbus, it's an engineering marvel, but there's no sense of passion" -- J. Clarkson re: Coxster