Joe Thonrton - 100+ point player in his prime, hit 80 pts once from age 31 and beyond
Pavel Datsyuk - 87+ pts in four straight years in his prime, never scored over 70 pts again in his career after 31 (had more injuries but his production rate fell off too)
Bergeron - Pretty much the exception here but still hasn't hit more than 70 pts since early on his career, he makes a living off being the best defensive forward in the game and netting 50-65 ish pts every year
Getzlaf - I think its safe to call him an 80+ pt player in his prime (happened nearly four times in 7 years in his prime with one of those years being 76 pts in 67 games) and has not eclipsed 75 pts once since turning 30
if anything, you just gave four examples reinforcing my point that a dropoff for Couture is liked and to be expected by the time he is 33
Couture will likely decline to some extent but he will still be a quality contributor which is all that the team cares about. If he can't play the pivot at some point, they will move him to wing like they did with Pavelski. The Sharks aren't going to have a problem filling spots when they need to. It's part of the business.