"Hacker" may very well be (helpfully) keeping the size of HN down. The last time I sent someone a link to an HN discussion they took one look at the url and phoned to tell me that my email had a virus.
Probably because it allows easy group identification. Those that use the positive meaning are in the group and those that use the negative meaning are outsiders.
We use many words this way. At the risk of sounding very suburban, witness the difference between Gangsta/Gangster when used by people inside/outside hip-hop culture.
Any language is, but not by simple majorities. In practice there is something like a pagerank for people for language usage, just as there is for most topics.
For example, it could be that 99% of people who use the phrase "begs the question" misuse it. But the 1% who don't are not merely a random 1% of the population. So in disputes about how to use the phrase, those who get it wrong tend to defer to those who get it right. You couldn't say the meaning of the phrase had changed till that stopped happening, because it implies by induction that everyone who misuses the phrase would change their usage if called on it.
Old meanings are not considered wrong of course, but are often pushed back to the higher-numbered definitions and eventually considered an old or archaic usage.
It doesn't work the same in all languages; for example, French has the l'Académie française which strongly attempts to regulate the meanings of French words, and is quite successful.
To prove my point: there are two official meanings of "begging the question," including the one you assume to be incorrect in your post:
http://www.merriam-webster.com/dictionary/begs
Furthermore, I'm confident that with some research, we could find several words and phrases in your post which were considered "incorrect" several hundred years ago (despite being commonly used then) but are considered "correct" today.
Nor is the shift of meaning in a word over time identical with people misunderstanding the previous meanings. That type of shift is quite rare. It's much more common for words to shift meaning the way "computer" has than the way "buxom" did.
There's nothing official about dictionaries, incidentally. They describe usage (as well as they can); they don't prescribe it. If dictionaries defined terms in the prescriptive sense, I could refute you by starting one.
Assuming that a shift in a word's meaning due to "incorrect" usage is rare, what makes it any less valid of a change? And are you willing to have the same view regarding all English words that turn out to have been formed as a result of misuse that you have toward "hacker" and "begs the question," even if you are forced to use an obsolete word?
You argue that dictionaries can't prescribe language, and I agree. I'm saying that there are limits to how much you can prescribe language in general, including the attempt to prescribe the meaning of "hacker" and "beg the question." The "beg the question" of 2009 is no less valid than the one of 1600.
There are more extreme cases of this kind of silent dismissal: thick brooklyn accents, ebonics, southern accents, spanglish, etc. will not get you very far in many places I've worked. I'm not saying that's morally right, but it certainly happens.
I don't hear hackers used for fear mongering as much anymore, so I suspect that hackers will eventually reach the top of the hill. Of course, I expect it will always have slight negative connotation.