Before 23/24, I think most had those two players in the mix for 2nd best player in the league with Draisaitl and Matthews (among forwards). By their late 20s, neither had hit a level that would have placed them on the same tier as Crosby, Malkin or Ovechkin (the post lockout era), or among others like Jagr, Forsberg, Lindros, Sakic, Yzerman, Lafleur etc.; players who had higher offensive peaks and/or higher all around peaks.
Last season certainly was unexpected by both; a bit moreso given their age and career stage. Kucherov seemed to have peaked in 18/19 and MacKinnon maybe had one Art Ross if he hadn't kept missing a few games every year but there wasn't an indication he had a really dominant season like 23/24 in him.
Since 17/18:
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McDavid - 1.61
Kucherov - 1.46 (#4 in points)
MacKinnon - 1.42
Draisaitl - 1.35 (#2 in points)
Panarin 1.24
#10 - 1.17
#25 - 1.00
MacKinnon and Kucherov stand out as #2/#3 (arguably clearly if you think that Draisaitl's numbers are likely a bit inflated due to McDavid.) with a clear gap between them and the pack. Both have playoff numbers (points/PPG) befitting their regular season dominance.
From 06/07 to 13/14:
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Crosby - 1.42
Malkin - 1.22 (#5 in points)
Ovechkin - 1.18 (#1 in points)
St. Louis - 1.07
Thornton 1.06
#10 - 1.01
#25 - 0.90
The clear #2/#3 are Malkin and Ovechkin, whose status is boosted with his GPG, with a clear gap between them and the pack. This time stretch includes Ovechkin's two "down years" and some injury-affected seasons for Malkin.
From 95/96 to 03/04
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Jagr - 1.38
Forsberg - 1.30 (5th in scoring)
Sakic - 1.20
Kariya - 1.09 (7th in scoring)
Palffy - 1.08
#10 - 1.00
#20 - 0.94 (adjusted for league size)
Forsberg and Sakic stand out as the clear #2/#3 and they add impressive playoff numbers to boost their status (i.e. they are better playoff performers than MacKinnon and Kucherov, not clearly but enough to place them above IMO).
Comment
While they have made a statistical case, primarily, if not exclusively, on their 23/24 regular seasons, intuitively, it doesn't feel like MacKinnon and Kucherov have placed themselves in the discussion with players who are Top 30ish caliber all-time in terms of peak/prime.
There is certainly a question as to whether the increase in league scoring since 17/18 has opened the door for superstar talent to exploit, perhaps even more noticeably in the playoffs.
I think McDavid has separated himself over his career offensively on the same level as Crosby, Jagr (for 7 seasons), and Howe. Howe is a great comparable as one can argue that his 06 era peers- Beliveau, Hull, and Mikita, matched his 2nd, 3rd and 4th best seasons but only once. McDavid's immediate peers got close to his peak once while he has two other dominant seasons on his resume. The question is, are MacKinnon and Kucherov on the same level as Beliveau, Hull, and Mikita.