As has been pointed out, assessing quality of the league across different decades/eras becomes a tangled web. To rely solely on your eyes to tell you the difference is not so reliable and almost inevitably leads to disagreement. It's one thing to say the average (median) player is more athletic, and another for that to prove that the best player is better than just 10-20 years ago.
For one thing, we can't assume that the distribution of talent within a league is not significantly different in some respects across eras. For another, although I agree that the total amount of talent above a certain threshold increases generally over time, it does not do so evenly for at least a couple of reasons.
First, it's more important that the number of teams is kept in rough proportion to the total talent. When the league is suddenly expanded rapidly like in the 70's, the talent is quickly diluted and therecan be a greater disparity that allows the best to exploit the worst. While when there is no expansion for a long period, the talent tends to compress as the number of teams does not keep up with population growth, as happened in the 60's and somewhat in the late 80's and recent years.
Second, there are other factors that have a dramatically greater impact than population growth in the short term. These factors include wars, league competitors and mergers (e.g. WHA), expansion or contraction of # teams, and the addition or subtraction of large talent pools (e.g. overseas talent).
Finally, even when other factors may be constant, but talent increases over time with population, the distribution of talent is not consistently smooth over time. Let's look at some peaks and valleys of more recent distributions of skaters, focusing primarily on offensive talent. Starting with the group which dominated much of the 70's:
Jan. 1948 to Sep. 1951 (~3.7 years)
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1948- (Orr), Kharlamov, (Park), (Lapointe), Robert
1949- Clarke, Goring
1950- Perreault, Sittler, Leach, MacLeish
1951- Lafleur, Dionne, Martin, (Robinson), (Salming)
Oct. 1951 to March 1960 (~8.5 years)
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1952- Barber, Shutt, McNab
1953- (Potvin), McDonald, Middleton, Gainey, Lysiak
1954- Gillies, Simmer, Gare, Greschner, Bourne, Rogers
1955- Larouche, Maruk, Paiement
1956- Trottier, P. Stastny, Federko
1957- Bossy, (Langway), J. Mullen, Tonelli, Duguay
1958- (Fetisov), Bo. Smith, Linseman, Smyl
1959- Gartner, Propp, Mats Naslund, A. Stastny, Broten, Ogrodnick, Vaive
Some fine talent, but Trottier, Bossy and Stastny stand apart from the other forwards.
Feb. 1960 to October 1965 (~5.7 years)
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1960- (Bourque), Kurri, Goulet, Ciccarelli, Anderson, Steen, Kerr, Hunter, Larionov, Carbonneau
1961- Gretzky, Messier, (Coffey), D. Savard, (Murphy), Nicholls, Larmer, Pederson
1962- Oates, Br. Sutter, G. Courtnall
1963- (MacInnis), Francis, Gilmour, Hawerchuk, Andreychuk, Thomas
1964- (S. Stevens) Br. Hull, Bellows, Tocchet, Verbeek, Sandstrom, Ferraro
1965- M. Lemieux, Yzerman, LaFontaine, Neely, C. Lemieux, K. Stevens, Tikkanen, Ronning
Even if you take Gretzky and Lemieux out, this <6 year period still stacks up pretty well against the 50's list of >8 years. Then comes a short drought of elite talent:
Nov. 1965 to Dec. 1968 (~3.3 years)
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1966- Robitalle, Nieuwendyk, Roberts, Muller, Richer, Mellanby, Olczyk, Sheppard, Pivonka, Clark
1967- Damphousse, Janney, Young, Dahlen, Simpson, King
1968- (Leetch), Recchi, Fleury, Bondra, Graves, Carson, Malakhov
Then another boom period, in large part to Europeans and Russian republics:
Jan. 1969 to Dec. 1973 (< 5 years)
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1969- Sakic, Shanahan, Fedorov, Mogilny, Turgeon, LeClair, (Blake), Khristich
1970- (Lidstrom), Selanne, Modano, Roenick, (Zubov), BrindAmour, Amonte, Guerin, Zhamnov, Lang
1971- Sundin, Bure, Weight, Holik, Nedved, Reichel, K. Primeau, Rucinsky
1972- Jagr, Alfredsson, Tkachuk, Palffy, Whitney, Nolan, Sl. Kozlov, Straka, Nylander, Stumpel
1973- Forsberg, Lindros, (Niedermayer), Kovalev, Mk.Naslund, Yashin, Stillman, Rolston, Lehtinen
Then another trough:
Jan. 1974 to Dec. 1978 (< 5 years)
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1974- (Pronger), Kariya, (Gonchar), Arnott, Demitra, S. Koivu, Sullivan, Satan
1975- St. Louis, Bertuzzi, Prospal, Langenbrunner, Morrison, V.Kozlov, Allison
1976- Elias, Hejduk, Smyth, Doan, Langkow, Sykora, Drury
1977- Iginla, (Chara), M. Savard, Briere, McDonald, Dvorak, Hecht, Handzus
1978- Datsyuk, Jokinen, Samsonov, Dumont, Zubrus, Huselius, Horcoff, Cole
And so it goes...
Really the only objective way I can think of to measure how quality increases/decreases between eras is to look at a large sample of the top talent and see how they performed at different times. I'm in the midst of a longer term project examining the better point scorers since WWII (hundreds of players), but am currently bogged down in the mid-late 80's. An easier if less reliable study might be to look at top goaltenders and how their adjusted GAA differed over the years. Otherwise it all comes down to opinions.