It's been explained correctly here several times. "Could" is not right in this context; it's often used wrongly like "would of", "supposably", "irregardless", and many other words and phrases. Frequent incorrect usage doesn't make it right.

But being in the dictionary (which both phrases are) does make it correct. And formerly incorrect usages become correct usages over time through frequent use, so you can't dismiss 100% the frequent usage factor.

See my comment #234 above.

Yeah, except they're in the OED. Both phrases are. Nice try. It's a difference between British and American usage. See, what I don't get is that when people correct British spellings to American spellings, they're immediately dismissed and yet with this phrase people continue to argue back and forth. Double standards if I ever saw one. And with regards to your comment "The meanings of the words themselves have not changed no matter how many people wish it were so", you're quite simply wrong. Or are you going to tell me that the word "gay" means "happy" and not "homosexual"? Or that the word "nice" means "foolish, silly, simple; ignorant"? You accept the modern meaning of those words (at least I'm speculating) so you are by your own definition "dumbing down the language".