I don't think that they "ease up." I think that deep in the human psyche the motivation can be intense if you are chasing a sacred goal and the let down, not intentional in Skinner's case, can be steep after you reach that goal. $70,000,000 is a lot of motivation. Does it outweigh the desire to be the greatest and win the cup? In some yes and in some no.
Jack Eichel seems driven by a desire for greatness. But maybe he meets a beautiful women, falls deeply in love and only cares about her and their kids, his hockey suffers for it. That's just human, too.
Playing at a top level means thinking about the game a lot, and working on perfection. Life changes things so maybe a player just doesn't focus as much anymore on the little things. Skinner, I would argue, lost some motivation after reaching his dream goal.