bugs don't transfer, fixes don't transfer. this isn't retail, they don't update the server, they create a new one. its why ragnaros is being shut down instead of being updated to legion, and why character transfers are required. the legion core could have less bugs or more bugs, it could have some bugs we had in wod fixed, but have some bugs we didnt previously have.
so if they just kept working on wod until they fixed it and then they closed it, they'd be doing nothing but wasting time. the sooner they get to work on legion the sooner they can fix bugs on that and make it playable. who knows, maybe pre-wod dungeons and raids will all be working now (cata and mop mostly, others work fine-ish already on mop)