That’s not really a statement that exclusively applies to Jagr. That the league went through massive changes twice in just 7 years means that all of the truly great players around Jagr’s age (Sakic, Brodeur, Lidstrom, Forsberg, Selanne) also had great years in all three of the high-scoring 90s, Dead Puck Era, and post-lockout 00s. More than that, when the scoring levels dropped again in the 2010s, Selanne added a top-10 scoring finish, Lidstrom added a Norris Trophy, and Brodeur went to the Finals.
It definitely leaves them with some battle-tested careers where you know they would be excellent in any era, but are we really uncertain that the older group (Lemieux, Roy, Messier, Bourque) who were great in the mid-80s, early-90s, and Dead Puck Era wouldn’t also be great in a post-lockout environment, had they been 5-10 years younger and saw three distinct environments too?
Just because Brett Hull at 41 fell off in 2005-06 doesn’t mean that being great in 2003-04 and 2005-06 is some unimaginable feat. Whether you look at forwards, defensemen, or goaltenders, the best ones were still generally the best ones.