That leaves out the sizable kind of people who actually are introverts. It strikes me as rather dangerous to take what seems to be a pretty simple observation - people occupy a spectrum from 'enjoying little to no social contact' to 'does not enjoy being alone at all' and we call this introversion/extraversion - and basically arguing that one of the end of the spectrum is 'wrong' and can be fixed. Not to mention the fact that most actual research on the matter seems to indicate that introversion is, at least to some significant degree, innate, and at the very least can be distinguished at such an early age that it might as well be innate.
Honestly, it's not so different in my mind from believing gay people are gay because <insert some self-serving theory> instead of accepting that in lieu of full understand, we should maybe just assume they are actually gay.
Or Christians insisting that non-believers are in denial and even actively rejecting 'the Truth', actively unhappy, and that they have gaping God-shaped hole inside of them.
Or, for that matter, Christians insisting that other Christians are not happy or doing well because they don't have enough faith, are not trying hard enough, or because 'there must be some kind of sin in their life'.
The danger of such statements is that they have massive impact on the group your judging, and that they ultimately can never be fully refuted. What makes it worse is that, in my experience, most 'outliers' - whether gay, autistic, clinically depressed, etc. - already live with a permanent suspicion that they're just not trying hard enough and making excuses.
EDIT: to be clear, I do agree that any of these labels can be used as an excuse, and nothing is 100% fixed. So in context you might be right. I just feel it's important to note that there are many people who are truly introverted, and they should not feel bad or wrong about that.