Not to point you out, as other have claimed the same thing but how could it not be more difficult in a 30 team league and with elite talent say from 30-40% from none Canadian sources since the early 90's than say in 1952?
Simple math will tell us that variance is much more likely in a 30 team league than a 6 team one all with top line and PP opportunities for each team.
Sure getting a calculation would be difficult and maybe even pointless and it wouldn't be perfect by let perfection hold us back from an improvement on the current let's judge guys how they did against their peers approach, which implies inherent equality through out time?
i haven't fully thought this out yet, so i'm only going to tackle the larger team league part, not the expanded talent pool part. but i always think of phil goyette when i think of expansion.
26 years old in 1960, should be near his physical peak, probably should be as good a player as he's ever going to be, and he's a (very good) checking line center on the habs dynasty, putting up good points for that role but in no way, shape, or form challenging for art rosses.
a guy who is about the same age, norm ullman (like goyette, also a bit of late bloomer), already has a top ten finish in 1960, and through the '60s will go on to place in the top ten the majority of seasons.
now by any objective calculation, norm ullman was a far greater player than phil goyette. never won a cup and never played a dynasty, but a very good playoff record when his teams were competitive.
in a 6 team league, the cream rises. ullman is a scoring star, goyette (even as the top offensive option on a weak rangers team and hitting the top ten once pre-expansion) is solidly behind ullman up to expansion.
after the first wave of expansion, phil goyette, 36 years old, finishes a mind-boggling 4th in league scoring. so yeah, that's the variance you were talking about. far and away his best year ever. ullman, on the other hand, is a steady top ten guy. peaks at #2, but i think as a general rule is #6 in 1961 and is #6 in 1971. the true HHOF guys like ullman are who they are. whereas goyette clearly benefits from expansion. so it's "harder" for an ullman to dominate his competition post-expansion because you get spike years from your phil goyettes and red berensons. and the gap between a goyette and an ullman, looking just at their overlapping post-expansion careers, is misleadingly close, though obviously ullman still is and looks better. so by the same token as competition is "harder" for ullman post-expansion, it's also "easier" for certain other players, because now there exist the opportunities for them to succeed as number one options.
this is radically simplifying it, but here's a quick and dirty breakdown:
types of players disadvantaged by expansion
now when we talk about the best of the best, the advantage i'm describing above doesn't apply. ray bourque is ray bourque in any league. and by this logic, ullman is ullman in any league. they are both guys who would rise to the top in a six team league, and who if the league expanded would still be at the top. all that changes is the extra expansion-aided spike years they have to compete with.
types of players unaffected by expansion
but to be precise about it, when we're talking about the absolute best of the best howe, mario, or gretzky, the extra expansion-aided spike years also don't apply. no pat lafontaine spike year can compete with mario, even when he misses 30% of the season. no peter stastny career year can touch gretzky, and i firmly believe that no imaginary peter stastny career year could have touched peak howe in the early '50s.
types of players advantaged by expansion
but where expansion does advantage players is, to take one your examples hardy, a guy like ziggy palffy. palffy was not bure, he was not selanne. those guys would have been first liners in a six team league. palffy was wonderful and he was one of my absolute favourite players of the early 2000s, but a guy like that probably gets buried on an O6 team. he's not as good as richard, geoffrion, howe, bathgate, and so on. so when we come to talk about guys like palffy or pierre turgeon or nik backstrom (the center one), or olie kolzig for these top positional lists, and when we grouse about palffy not getting the look that an O6 guy like dickie moore will get, or olie kolzig not having the "opportunity" to succeed that gump worsley did, one would have to also ask the question of whether any of those contemporary guys would even have cracked the NHL during the O6, and if so how far down the lineup they would have played.