blizzard original plan for van wow was continuing content of van wow thru patches, KARA, COT, more instances etc. based on what was happening in van wow with gear gaps like 60's at MC vs god mode 60s at AQ/Naxx+ gear, continuing and adding to van wow "would create super 60's" that most would never catch, balancing issues, etc. they decided to go with the expansion method, this sorta gives players a clean slate to catch up to current content vs being years behind in old content.