I've never seen any compelling evidence to support the idea that Gretzky's goal scoring declined at a different rate than the rest of his offensive performance, which is what would have happened if he was truly unable to adapt his goal scoring to a different environment.
The theory that Gretzky declined as an overall offensive player after the Suter hit is obviously better at explaining the relevant data.
If you look at every season of Gretzky's career, and calculate the percentage of his even strength points that were goals, the lowest was naturally his last, where he scored just 6 even strength goals in 70 games. The season with the second-lowest percentage, though? Surprisingly it was 1985-86, the year he hit his career high in points. How does that make any sense, if old Gretzky apparently couldn't score on the newfangled butterfly goalies? It actually turns out that his rate of even strength points that were goals was surprisingly stable over his career after 1984-85.
It was a similar story on the power play, where at age 24 in 1984-85 Gretzky was already at only 18% of his PPP being PPG, reflecting his usage changing from being a shooter to a outright playmaker. He would maintain that role for the rest of his career (from 1986 to 1999 combined his PPG/PPP rate was 19%).
See this chart of Gretzky's ESG% and PPG% from age 25 (1985-86) to age 37 (1997-98), excluding his final campaign where he did legitimately decline in terms of goal scoring. The trend line for his even strength rate is perfectly flat. Again, if there was something about the improving league defensive structure or improved goaltending that impacted his goal scoring more than the rest of his game, that simply would not be the case, especially given that these major changes mostly took place when Gretzky was in his early thirties.
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Gretzky was already an out-and-out playmaker by the mid-'80s, and he continued to be the same type of player for the remainder of his career. He was just so good at creating offence, and league scoring was still relatively high, that he ended up with a lot of goals anyway. But when he got Sutered and league scoring started to drop, it had a huge impact on his overall scoring totals, but certainly not his goal scoring uniquely. This is why it is a misinterpretation of the data to say that he couldn't maintain his goal scoring (he did in fact maintain his goal scoring just as consistently as he maintained any other aspect of his offensive game).
If you want to question any part of Gretzky's goal scoring resume, question whether his peak would translate to other eras, because there you actually have good arguments to bring to the table (the "Bossy was a better goal scorer than Gretzky" thread has most of the good ones).
In contrast, if you want to know what it looks like when a player actually hits a wall in terms of even strength goal scoring, Mario Lemieux is a great example. Mario had a super hot streak of 10 even strength goals in 13 games to start off the 2000-01 season, and then he fell off a cliff for the entire rest of his career:
Year | Type | Age | GP | ESG | ESA | ESG% | #10 in NHL | Adj ESG | Adj ESG/GP |
1996 | REG | 30 | 70 | 31 | 43 | 42% | 0.341 | 27.3 | 31.9 |
1996 | PO | 30 | 18 | 7 | 7 | 50% | 0.341 | 6.2 | 28.1 |
1997 | REG | 31 | 76 | 32 | 47 | 41% | 0.341 | 28.1 | 30.3 |
1997 | PO | 31 | 5 | 3 | 1 | 75% | 0.341 | 2.6 | 43.2 |
2001 | REG (1st 13 GP) | 35 | 13 | 10 | 6 | 63% | 0.317 | 9.5 | 59.7 |
2001 | REG (Rest) | 35 | 30 | 6 | 19 | 24% | 0.317 | 5.7 | 15.5 |
2001 | PO | 35 | 18 | 5 | 5 | 50% | 0.317 | 4.7 | 21.6 |
2002 | REG | 36 | 24 | 4 | 13 | 24% | 0.317 | 3.8 | 12.9 |
2003 | REG | 37 | 67 | 14 | 32 | 30% | 0.293 | 14.4 | 17.6 |
2004 | REG | 38 | 10 | 1 | 4 | 20% | 0.268 | 1.1 | 9.2 |
2006 | REG | 40 | 26 | 4 | 12 | 25% | 0.293 | 4.1 | 12.9 |
1996-Jan 2001 | Both | 30-35 | 182 | 83 | 104 | 44% | | 73.6 | 33.2 |
Jan 2001-2006 | Both | 35-40 | 175 | 34 | 85 | 29% | | 33.8 | 15.8 |
For Lemieux, we see not only his overall rates drop, but his percentage of goals drops from 44% to 29%, something that never happened to late-career Gretzky other than during his final season. Era-adjusted, those last 175 games for Lemieux are barely better than old Gretzky (age 35-38, Gretzky had 14.7 adjusted even strength goals per 82 based on the same calculations as above).
The "Mario was better at adapting as a DPE goal scorer" seems to handwave away a lot of evidence (particularly playoff evidence), while really boiling down to a theory that is almost entirely based on a half-season of play in 2000-01. It's probably still a defensible claim to some degree, but I think it's vastly overstated in most cases and definitely not something that gets Lemieux even remotely close to a GOAT case.