To clarify, I don't challenge any Art Rosses or Harts from the O6. They, like any other Art or Hart wins from any other era, can be reasonably measured against any other elite seasons.
I don't think the expansion in '67 proves anything really. It wasn't like the # of potential star players doubled overnight. But it is reasonable to conclude that there are more elite level players 50 years after that occurred given population growth and with the intro of more players from outside of Canada. It should be generally accepted that the league needed to grow in size to accommodate the growth in NHL quality players. Whether 31 teams or 20 teams or 40 teams is the right number should be somewhat moved to a secondary level of discussion, and completely outside of a statistical one.
So if you are able to finish in a higher percentile out of a group of peers who were just as talented as the group of peers 50 years ago, that should hold some value.
Any micro comparison, e.g. a 3rd or 5th place from 2017/18 vs. a 3rd or 5th place from 1962/63, needs context, context that may put one clearly over the other, but when you are talking numerous Top 5 or 10 finishes, I think you can make a case that numerous Top 5 finishes from the current era > the same number from the O6, or at the very least not accept that a Top 5 from the O6 automatically trumps a lower finish from the current era. Crosby's goalscoring is an example of this. He lacks numerous Top 5 goal or GPG finishes in comparison to top goalscorers from the same timeframe in the O6 but still has an argument for #2 goalscorer of his era, albeit a weak #2 in relation to other timeframes.
I think Hull, Beliveau and Crosby all clearly stood out from their peers at a level that is as good as any other non Big Four player and for a notable extended period which sets them apart from those players. Perhaps their isn't a strong case to be made for differentiating their top scoring and Hart finishes as they all had many Top 3 finishes. I can see that it may take more Hart nomination level seasons by Crosby to clearly separate himself from those two.
I think where things get interesting is comparing Crosby's top scoring finishes with Howe's, who had quite a few 5th place finishes, a level where the statistical differentiation between now and the O6 start to clearly establish itself. I would make an argument that, generally speaking, a Top 10 finish from the current era is equal to a 5th place in the O6, and that a Top 5 from the current era is equal to a Top 3 finish from the O6.