I do understand your points but disagree with the conclusion drawn from them. Maybe I'm being pedantic but the conclusion I pull from them is:

Communities are noisy, and a good chunk of activity is overrated. But not the community in and of itself.

In a well-established and managed communities, the signal may be significantly smaller than the noise BUT there are mechanisms in place that make sure signal rises high above. This forum does okay but with a few tweaks could do much, much better (weeding Off Topic posts from Active Topics was one valuable step toward that).

So in the end, even 95% noise shouldn't matter and certainly shouldn't devalue any community (much less the broad concept). The trick is to let noise be noise and float the cream to the top over it.