It seems to me we are talking about two things here, one of which is correct (if rather obvious and didn't require a thread) and one of which is highly debatable:
1) Gretzky's goal-scoring declined from his 1981-82 through January 1987 peak goal-scoring period. (We already know this, as anyone can look at the stats.)
2) The reasons Gretzky's goal-scoring declined. The issue I take with the thread premise is the argument that the goal scoring decline happened because of better overall League defense, goaltending, depth, etc.
Let me briefly deal with each of these points, and then I'll be done with the thread... until Stanowski's next one. (Just kidding! Maybe.)
1) Gretzky's goal-scoring volume definitely did decline from around February 1987 through the end of his prime, September 1991.
During Wayne's last 1.5 years in Edmonton and his first 3 years in L.A., he ranked as follows in goal-scoring:
-- Total goals = 9th
-- Even-strength goals = 6th
-- Goals-per-game (min. 200 GP) = 8th (0.59 GPG)
In terms of goals-per-game, Wayne in this period is well below Mario, Hull, and Yzerman... but that's it. He's on par with Neely, Nieuwendyk, Robitaille, and Lafontaine. (I'm too lazy to calculate ES goals-per-game, but from a glance he's behind Mario and Hull and Yzerman, but he might be 4th.)
So, before we get into WHY he declined from lapping the entire field by record amounts in goals to being only the 5th or 6th or 7th-best goal scorer for the last 4.5 years of his prime, let's pause to keep one thing in mind: He was a playmaking center who was injured three times and traded once in his period. If you disregard his goal-scoring prowess from 1981-82 through January 1987, his Feb. 1987 to Sept. 1991 goal-scoring is remarkably good from this considered perspective. (But that's getting a bit off-topic.)
We should also bear in mind about this period:
1986-87: Gretzky was 1st in goals
1987-88: Gretzky was 2nd in goals on Dec. 30th when he got injured.
1988-89: Gretzky scored 50 goals in 66 games
1990-91: In the first half of the season, Gretzky scored 25 goals and was third in ES goals (during his fall-off goals-wise in the latter half, he had 72 assists and L.A. was the #1 team in the NHL).
So, during this period of relative slow-down goals-wise (while still being an elite goal-scorer), Gretzky was still capable --- as in 1985-86 --- of "turning on and off" the goals-tap, to some extent.
2) The reasons Gretzky's goal-scoring declined.
Gretzky's goal-scoring slow-down (down to the same levels he was at in 1980 and 1981) from Feb. 1987 to Sept. 1991 was NOT due to NHL defense, player depth, or goaltending.
As has been shown, Gretzky at any given time in this period (first half of 1987-88, first 66 games of 1988-89, first half of 1990-91) could jump up to the top of the League's goal-scoring ranks. This wouldn't be possible over fairly long stretches of 35 or 66 games if he were being limited by an overall better NHL defensive game.
Further to the point, 1986-87 was the lowest-scoring season of the entire 1980-81 through 1989-90 period (one player -- guess who? -- scored more than 107 points), and Gretzky won the scoring title by the largest margin of his (or anyone's) career and led the NHL in goals.
Again, in 1988-89 Gretzky (on a new team, at that) scored 50 goals in 66 games. This is the final (or next-to-last, depending on how you do it) season of the 1980s, and this goals-pace through 66 games was better than he'd done in 1980-81 or 1985-86, and about the same as he'd done in 1982-83 (when he scored 71 goals in total). So, if better NHL defences was slowing down his goal-scoring, I'm not seeing it...
Then, what about goaltending slowing him down? As I've already shown earlier in the thread, Wayne's shooting-percentage his last three seasons in Edmonton and his first three seasons in L.A. are basically identical. Considering that NHL goaltending was indeed getting better, and considering that he was transitioning to be more of a perimeter attacker in this period (i.e., more shots from distance; fewer from close range), this actually suggests to me that he was better at beating goalies at the end of the 80s/early-90s than he was from 1985-86 to 1987-88.
In fact, in the first half of 1990-91, Gretzky's shooting percentage was higher than Brett Hull's, who was on the way to an 86-goal season.
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So, if Gretzky was doing all this, why did he fall to being the 5th or 6th or 7th-best goal scorer during this 4.5 year period that ended his prime? Two reasons:
1) Higher high-end competition from Mario and Brett Hull. In any scenario, the best goal-scoring finish Wayne could have hoped for in this period was third, as Mario was right at his peak and Hull was just reaching his. Those guys' peak seasons are better (in goals) than any competition Wayne had from Bossy or Kerr or whoever. (Now, of course, if Wayne had been 21 or 23 in this period, it might be a different story...)
2) Wayne's drop-off later in seasons. This might be related to aging, or fatigue, or maybe just his not caring as much about goals-totals later in seasons anymore (I don't know), but there's a definite pattern going back as far as 1984-85 when Wayne's goal-scoring dropped-off during the final 10 or 20 or 30 games.
1984-85: First 50 games = 51 goals / lLast 30 games = 22 goals
1985-86: First 45 games = 35 goals / Last 35 games = 17 goals
1986-87: First 49 games = 48 goals / Last 30 games = 14 goals
1987-88: First 37 games = 30 goals / Last 27 games = 10 goals
1988-89: First 66 games = 50 goals / Last 13 games = 4 goals
1989-90: (Exceptional season)
1990-91: First 40 games = 25 goals / Last 38 games = 16 goals
The only season later in his prime that isn't still quite impressive from a goal-scoring perspective is 1989-90... he was quite consistent all season, but never reached an elite goal-scoring level (still scored 40 goals in 73 games, though).
That is all.