I doubt WDW monorail fleet will be updated any time soon. The original trains ran from opening into the 1990's, so the current mark VI fleet has only been around for a little over 10 years. The purpose behind the monorails are different for the two parks as well. DL is still more of a ride than necessary transportation, where as for WDW there is only two ways, (walking from the CR not included), to get to the park. It is either a boat or the monorail. Which is why when the WDW fleet was updated, they added more cars to the train to increase the overall capacity.