Byran Little averages 45 points each year. Last year was no different...Little scored 46 points.
Thomas Vanek on the other hand is a solid performer. Vanek averages 64 points every year...last year he scored 61.
Evander Kane on the other hand scored 26, 43, 57. Statistics show players are more productive at 23 to 26. I'll predict Kane scores 68 points next year.
At 21, his first full season, Little scored 31 goals and 51 points. The next season he scored 13g, 34p. The next 18g, 48p. 24g, 46p last year. Similar amount of games each year.
At 23, in his 2nd year, Vanek scored 43g, 84p. In the five years since, he's averages 32.4g, 63p. Again, no major time lost to skew the averages.
Thus far, neither player has matched their early production. Little's goal scoring regressed substantially, just starting to rebound 3 years later. Vanek's point totals regressed substantially, and given his age now he may never match those totals again. Both players regressed during that 23-26 age range. Kane follows that path, you should expect him to average ~45 points for the next few years.
Bobby Ryan is another example, though he's mostly just stagnated. 31g, 57p in 64 games as a 21yo rookie (40g, 73p pace for 82 games). Fairly similar the next two years, then a down year this one, where he scored the same 31g, 57p but in 82 games instead of 64. Again, he's in your magic age bracket.
What most players do doesn't mean Kane is certain to improve. He might, and I'd love to have him. But even if he does score ~68 points next year, paying him $6.5-7M is still overpayment (unless 50 of those points are goals). Especially considering he's not a center nor particularly good defensively. And you have to give up a bunch of draft picks for that right. And that's making the assumption that you could even get him that cheap. The Jets would know better than anyone his potential for further improvement, and how much it would hurt their team to lose him. If they think there's a reasonable chance he can live up to a deal like that, they'd match.
There's a reason few offer sheets are made. It's not some code of honor between GMs. It's the RFA rules.