There are a number of erroneous assumptions, by both sides, in this discussion. I'll list and explain or clarify.
The O6 era is viewed as being the most competitive era in the history of hockey. Competitive at various levels - winning team and individual awards/honours and for playing jobs in the league.Competition for the bottom 10-20% of playing jobs during the O6 era was extremely fierce. Today this is not so. Even if the USA and Europe is factored in, replacing the bottom 70 - 140 players in today's NHL with quality is next to impossible.That is why so many 18-19 year old players will make the NHL next season.Do not equate competitive with best.
This brings us to the talent pool issue. The USA and Europe is a factor but Canada is still the major producer. Poor record keeping, lack of a fixed starting date, mergers, etc make it difficult to track the growth of Canadian minor hockey associations but many of the suburban ones that started in the post WWII era from scratch, maybe 40 - 50 teams(app 12 kids) have grown 7 - 15 fold(upwards of 500 teams,15 kids). Even balanced against inner city associations that have disappeared due to changing demographics there is a net gain in Canada that far surpasses the population increase.
Top players from 30 teams competing for awards or honours. Really? List the players from the non-playoff teams who were realistic competition for the AS teams or major awards that were not rookie based.Break it down even further amongst the playoff teams and you will be left with a serious pool of maybe 6 players per position.
The biggest issue is the bottom 10-20% lag. The bottom 2-5 players per team drag down performance. Prime example would be the Derek Boogaard type player - at least 30 of them in the league.Throw in the 1-4 cycle thru minor leaguers where the team hopes to get lucky and stumble across some production and you see the major weakness in today's NHL.
The main reason 18 and 19 yr old players are playing are due to the Cap not to lack of talent. Also there are plenty of "talented guys", usually guys who are either a little smaller or not the best skates in the world, who are a bit out of reach for top 6 postions which are today given to energy and role guys and tough guys.
The bottom lag doesnt really come minto play either look at the icetime for the bottom 2-5 players per team, maybe 10 minutes a game tops and usually less and certainly not any PP time and maybe only PK time if they are defensive specialists.
Also look at bottom guys like Ben Eager who would have been in the top 20% of skating ability in the league in the late 70's never mind earlier times.
The level of training and preperation that all players are involved in today also makes the gap from the top guys to the bottom 10-20% is much less than it used to be as well.
Bottom line is that you could take players who would not fit in a present verison of a 6 team league, ie players beyond the top 120 or so and place them with large roles on orignal 6 teams from the 60's.
I think alot of people get caught up in the memory and stories of the original 6 greatness and use terms like watered down 80's when Gretzky played (you can make a very strong arguement that with the inclusion of the 4 WHA teams in 1979 that it actually brought up the talent pool as the WHA no longer shared in trying to get players with the NHL. Players like Gretzky, Messier,Mark Howe,Cloutier,Kent and Ulf Nilsson, Mike Liut are a few on this list).
Expansion in 1967, 70, 72 and 74 might have had short term watering down affects but with the influx of players from overseas in the mid 70's and largley due to the influence of the WHA, any loss in the overall talent pool was quickly regained and indeed passed.
Just musing, but I wonder how we would have viewed Gretzky and other WHA stars if the WHA had survived and the NHL had remained a 17 team league, it sure would ahve been interesting.