I brought this up in the comments on the news article, as well as a few other people.
I agree that it's rather dumb to say the series has peaked because two sets of DLC and a handheld game didn't sell as well as a title that's as massive as GTAIV. No way in hell will they do as good as GTAIV. I think that seems like pretty common sense. Does that mean the series has peaked? Not even the slightest!
If, or when, GTAV comes out, I'm sure it's sales will match or even out sale GTAIV. There is no way you can claim a series has peaked because three 'side stories' didn't do as well as a major, numbered entry to the series.