That's a good point. I'd forgotten they did that for Overwatch. Didn't they also make a new engine for HotS? Or is that just the SC2 engine?
Either way, I agree that a clean slate wouldn't happen. One of the main reasons WoW is even still going as strong as it is is due to heavily entrenched player investment. Take that out of the equation and any kind of "WoW2" would have to be so incredibly, unbelievably good to make up for it.
Although I'm curious to see how that concept contrasts with Classic, which IS a clean start.