Well, there's multiple ways of looking at it. (And we're just talking about this season. I agree with you that they have shown signs of decline last few years. )
1. Fact that Z and Dats have been playing since before lockout should if anything show that they are hanging in there while the rest of a league had a nice 4 months off. In EV Zug Zet was playing exceptionally well. Overall this year he's doing better than he was last year in terms of production (not so much with goals scoring I know).
2. Ok lets say Z and Dats had the advantage in the beginning of the season and were on fire and the rest of the league sucked. But now the rest of the league has finally caught up and Z and Dats are getting tired.. So technically it should be balanced at this point.. However Zetterberg is officially ranked 6th in assists in the league (an impressive feat for a 32 year old tired man). Meaning that other players simply aren't able to catch up? Or, he's somewhat still maintaining his assist production, albeit not as well as he was in the beginning of the season.
And I don't believe Z has "dropped off big time" at all. Him and Datsyuk are still the best players on the team. And last nights game with those three assists were a clear message that he's not "dead" yet. The captain had a big game. Personally I think he's running on fumes, but I give him and Dats credit for doing what they can with a subpar roster and linemates.
Yes. Let's not confuse their declining skills with a lack of effort. Both are giving everything they have. The issue is father time has changed their abilities, as it does to all of us. It doesn't help things that they are our only real scoring options 90% of the time. That is not on them, it is on management. But Z and Dat are the ones paying for it.