Sure, and to this point, there have been at least a handful of times when the 51% attack could have happened, as well-known, related mining pools could have combined their power to achieve the majority. It has been plausible before, and I'd imagine what you mention is the reason it still hasn't happened. It would destroy the coin, because who knows what happened while they had majority. There would probably be some kind of fork, and I'm sure the value would diminish. It would be messy for everybody.
