I'm with LR on this, however obvious - NN will go on.
Naturally it's sad that some popular, vocal individuals have felt they had to leave, or that some might even have been banned following warnings, but let's be clear on this, NN's stock in trade is the pictures posted by members and non-members first and foremost, while the forums are likely to be considerably less of a draw for the vast majority of the (as of just now on the front page) 2,713,335 members of this site.
Of the 140,000 or so members quoted on that page as actively logging into the site each day, it's apparent that there's only a tiny handful of that number who actually contribute to the forum on a regular basis, so I can understand why the site owner would wish to make changes such as enlarging the number and scope of forum topics if he/she feels it to be a good selling point, we'd all do the same wouldn't we?
So ask yourself, if you were seeking to maximise your site revenue by making the forum appear more attractive, would you wish to discourage what might be seen as negativity or unfriendliness in the forums? But how would you do it? Would you rely on it to police itself? Or would you react to member complaints in a firm manner?
For what it's worth, I AM sad that some have left, but as has been said before, NN will go on.