I would assume the decision makers would be aware of this survey and even if they weren't, their own in house research would probably be similar. So I would say the wheelbarrow is most likely to be retired and the race car is least likely.
EDIT: The cannon was retired despite it being somewhat popular so they might be removing pieces related to war. If I had to bet I would bet the battleship.
EDIT: After thinking about it more I make the top hat the least likely to be retired. The monopoly man who is the trademarked symbol for the game wears a top hat. It has become associated with Monopoly.
EDIT: After doing some more research I believe the battleship is most likely to be retired. The cannon and battleship were used in a different war board game.. When that game failed they were recycled back into Monopoly. The cannon has already been discontinued. I believe the battleship is next.