Let's look at Power Backs:
Lynch has played 7 seasons.
Alexander played 8 seasons (Came back for a 9th with the Skins who cut him after 4 games).
Brandon Jacobs played 7 seasons (Came back for an 8th and left the Whiners after 2 games).
Michael Turner played 9 seasons, came back for a 10th this season, but was cut.
Tiki Barber played 10 seasons, but carried a bit less weight than Beast Mode.
Lynch turns 28 in 2 Months.
He's no Spring chicken for his position, had some knee issues this season and his production dipped a bit.
You look at who the Hawks have to re-sign in 2016, and I think it's clear that next year is Beast's final go 'round with Seattle.
I wouldn't be surprised if they drafted another RB since Turbin and CMike haven't really been able to push Lynch.
I don't think he'll peter out at the end like Willis McGahee or Maurice Jones Drew who are hanging on for 1-2 seasons too long. I think he will walk away with the $30 million or so he has earned, and if the Seahawks want to reward him, they won't cut him, they'll trade him to the Raiders after next season to let him go out after the dream of playing for his hometown team where he has his charities/community/family etc.
He is far from done and is still elite. But the end is near.