One means of estimating talent pools would be to go back to look at the youth registration records in the various countries. While this will not necessarily speak to NHL strength I think it can speak to what the skill level necessary. If there are 100K children playing hockey for a given age year and let's say, 60-70 openings for that are group in the NHL that will say something as compared to other similar numbers. Let us just assume that talent generally rises to the top.
The harder comparison will eras where "freedom of movement" and "tendency to move" is limited. Sure, the Euros play in NA now but they didn't used to... even now the KHL has an effect.
This may ignore the level of athlete who gets involved... you may generally have a stronger Canadian athlete pool because hockey is the thing to do... not so in the U.S. In either case, getting some metrics based upon age-group youth registration through the years may say something.
Will it? I'm not sure.
The issue with this approach is that in the "old days," most youth didn't register - they just went out to play in the frozen ponds.
I think it's also important to note not just the overall size of the talent pool (quantity) but also the (quality) as well.
For a number of reasons the number of Canadian kids playing hockey probably has decreased but the quality has increased.
Since the brawl at the 87 WJC between the Canadians and Russia there has been an increased focus on elite training among younger players.
In bC where I live, historically it has been a poor producer of NHL talent, in part due to lack of traditional "pond hockey" but in recent times it has joined the major Canadian producers of talent.
I think it's safe to say that the NHL has been changed quite a bit from the increased streams and available hockey pool (both in terms of quality and quantity) in the last 40 years.
