And it really was not even 5+ because the new team was not assembled until around 2008, and only scraps were developed up to that point.
Blizzard North was independently working on D3 when WoW was gearing up for release, the project failed on their watch as so much key talent jumped to flagship. The D3 we have was built from scratch a bit after they shut down blizz north, meaning only 5+ years of actual development.
4 Years for a fledgling game designer and little budget room will lead to garbage, we got to witness that first hand.
Things are changing within the company now, which will certainly lead to improvements.