I'd only consider it an error if the proper lessons aren't learned.
I think it has shown that the older version(s) of the game have a strong core, but adaptions at certain points are necessary because the community is a living thing.
To be fair, without a certain type of "mob mentality", we probably wouldn't have Classic to begin with because Bracks "You think you do, but you don't" would still be their final word on Classic servers, rather than being a prime example of how a company can be out of touch with their customers.
Classic's success is in itself proof that "Blizzard knows best" is faulty as well - massively sometimes in fact.