at what point does a word get used so commonly in an incorrect form that it becomes, literally, the new definition?
A slang synonym of "literally" is "very". It suggests something is so similar to the thing it's compared against, it ought to literally be that thing. It's a humorous exaggeration, humorous because the object in question is obviously not the other object.
No doubt, some users of this form are unaware of its original definition. Yet others are fully aware of the original definition and thus use it to upset pedants on Internet forums.
Language evolves and always will; does anyone think the English dictionary arrived on the scene mostly finished? If people understand your meaning, it must be some kind of language.
Of course, they can be used sarcastically or ironically or whatever so the meaning can't be unambiguous.