Kostitsyn played with Plekanec for long stretches, he wasn't always a 3rd liner.
Check his game log from 2010-2011, he had something like 6 goals in 9 games to start the season, then look after that.
http://espn.go.com/nhl/player/gamelog/_/id/2568/year/2011/andrei-kostitsyn
You can also see long droughts in 2012 and many people unlike yourself remember those droughts.
http://espn.go.com/nhl/player/gamelog/_/id/2568/andrei-kostitsyn
When he was demoted it wasn't only because he was in a scoring drought, it's because he completely disappeared. Kostitsyn was way too far inconsistent, not only in his production, which is not a big problem for a 3rd liner, but in his implication in the game.
I couldn't care less about his projected numbers, most of the time, the situation called for him being demoted, he had to be held accountable for his poor play.