I think this is an interesting question, but that the answer (is it age or games played that better explains/correlates with decline?) is probably somewhere in between.
Good post. I have to say, I don't think this is true. All players skills decline as they age and they need to operate under that view. Not only do their skills decline but it's harder and harder to recover from the slings and arrows of NHL fortune as you get older. And make no mistake, the NHL is a rough league and the NHL season is a long slog. Basically, players need to see missing a year through the prism of ,"it happened to Gretzky, it will happen to you." If they miss a year, then they will likely lose most or all of that income, especially the older, fringe players who will have to compete with the 1 or 2 classes of kids who will age into the league.
There is certainly a lot of research into physiological changes (metabolic, lung function, etc) that occur sometime after age 30. On the other hand, hockey (and probably football) players are fairly unique in their experience of "wear and tear" and its accumulation. So while the latter might essentially be "paused" during a work stoppage, the former most certainly isn't.
That said, what % of players actually make it past, say, 33 in the NHL? Presumably that's the group that suffers here, since they lose a year of play that can't be added on at the end of their career (or not with the same performance level), whereas a player who will eventually stop playing due to injuries (either directly, or because or diminished ability from accumulated wear and tear) probably just push that eventuality back a year.