I think it is a mix, soviet collapse and so on causing some drop, but also the aging generation being possibly the best generation of players.
What the stats show is that from '75 to '92, a time dominated by Canadians, a few Americans and a few Euros, the avg. age of the Top 10 was
25.2 years old and that there were, on average
4.3 U25s in the Top Ten per year.
From '93 to '04, there was influx of elite Euro talent, primarily Russian, that first squeezed out the young NA talent then squeezed out prime NA talent (age 25 to age 30) resulting in an increase in the avg. age of the Top Ten. The avg. age of the Top 10 was
27.5 years old and that there were, on average
2.6 U25s in the Top Ten per year.
During the DPE '97 to '04, there was a shift to older players and less young ones. Hard to say that this was due to older stars staying in the mix due to more talent among the aging stars or something to do with league dynamics (i.e. obstruction, more defensive, lower scoring) that allowed older players to stay more competitive with younger ones.
Also at the same time, there has been a clear shift in the overall league dynamics that sees older players stay in the Top Ten longer. Not a surprise given the superior conditioning from the '70s to today.
From '05 to '17, the avg. age of the Top 10 was
26.7 years old and that there were, on average
3.4 U25s in the Top Ten per year.
Since scoring has gone up starting in '18, the avg. age of the Top 10 was
26.6 years old and there were, on average
3.2 U25s in the Top Ten per year.
COMMENT: I don't think we can unequivocally say that the league during the DPE was any "weaker" or "stronger" where Jagr's relative dominance is questioned or heightened.
I am not a fan of making blanket claims like that anyways because we have zero clue how a player would do if they played in another era. That gets waaaaay to far into the "What if" weeds.
I don't mind giving peak Jagr an edge over peak Lafleur as I think the league was a bit more settled during his time (e.g post WHA, slower expansion) but I would not move peak Lafleur down a tier with players who were clearly statistically inferior.
Back to the broader point, the smell test sees objective analysis place players who overlap on reasonable statistical tiers.
McDavid's best is arguably the best of the post Wayne/Mario era but we saw enough overlap with Crosby to draw the reasonable conclusion that Crosby's peak is competitive (as it was with Jagr's for the same reason) but certainly is not head and shoulders above his immediate peers as Wayne/Mario and Orr were.
Howe's offensive best was the best of the O6 era, moreso than McDavid but was not on the same tier as Wayne/Mario and Orr.
Was Howe the Wayne/Mario of his era?
Was Howe the McDavid/Crosby/Jagr of his era?
I think the smell test places it in between those two.