Yeah, pretty much what they said. A language major of mine is always going on about how trying to correct people using words "incorrectly" is actually hampering the progress of the language, because, well, if the word starts being used in that way by the majority, then that word suddenly means a different thing doesn't it?
Meaning is assigned by the speaker, not the word. And the internet is doing a lot of shuffling around with it.
I disagree, in part. The lexical inertia of any given word is desirable, just as the ability of a language to mutate over time is. Without the prescriptive approach to language(i.e. how it ought to be used) it would quickly devolve into an innumerable regional dialects, making communication so much harder.
In your example, if every speaker could assign their own meaning to any given word without deference to the commonly accepted one, then we'd end up with people unable to talk to eachother due to endless semantic conflicts.
The insistence on "correct" usage by prescriptive linguists actually helps speakers communicate without having to learn the nuances of your conversation partner's dialect.
However, it is misleading to think that linguists only concern themselves with the above mentioned correctness. The study the descriptive approach(how people actually use a language) is on par with the prescriptive, as that is the source of change.
That change, as you've said, comes about after the majority of speakers starts using any given form, at which point it enters the realm of "correct" language and starts being enforced for the reasons mentioned before.