I don't know - Rakell was good for like three seasons until last year's resurgence (which is looking more like a dead cat bounce). Outside of 2016-2018, he is just a 2nd line winger, good for about 20 goals, 40 - 45 points per season. And I know some of those seasons were better than others if you games played into account, but other than the COVID year, those games played were down because of injuries, which should be factored in to his value.
So maybe it's not that his prime was short. He's actually still in it. But "prime" Rakell is a mediocre 2nd line right wing, when at his peak he was a decent top liner. His prime just isn't all that great. Not bad, certainly not for a late 1st rounder, but not anything to get excited about.