If you average out his career totals over 82 games, Boeser has averaged about 30 goals and 65 games. He slowed down after suffering a concussion but this year he is scoring at a 33 goal 67 point pace.
I have no idea how to accurately predict how a player would age. Like sure I can look at an average skater, player who took a lot of physical abuse, and or injury history and predict that he wouldn't age well but Kessel was an elite skater/goal scorer, an ironman, who rarely got hit and he didn't exactly age well. Work ethic can also go away. You can say that Lindholm had 1 offensively great season but he was good for a long while. The issue is that he was coming off a bad season so you don't know where his game was heading at age 30.
The truth is that a lot of Cup winners were in good situations. It's like if we won a Cup two years ago or last year where our core players were young and signed to good contracts.
If you don't want to sign guys like Boeser you need to make some homerun trades and signings to contend.
Yes, Boeser averaged roughly 30 goals and 65 points over his career so far, but that is in his peak and as a fixture on the top line with all the prime offensive opportunities and PP1 time. As he move past his peak, his scoring should naturally decline.
It is interesting you use Kessel as an example, even his last season with Pittsburgh he was a point per game player. His stats didn't fall off until he was traded to Arizona. Is the sudden and steep drop off due to his age, or is it due to an older vet not motivated to play for a rebuilding team? Work ethic was an issue with Kessel but it could also be said about Brock for a while.
I think a vast majority of the players decline as they are in the late 20s, moving into their 30s. There are always exceptions like Pavelski, and the elites like Crosby/Malkin/Ovi/Selanne usually remain productive, but the norm is that signing an aging forward long term is not a good bet to make.
Yes if the team is looking to contend, they will have to replace BB6 somehow. There are and will be options for scoring wingers though, there always are.