I actually think Lynch will probably play the contract out. Lynch isn't just a running back, if not for RW he'd be the face of the franchise. I know with the Mariners a lot of smart people have argued that trading Felix even for an amazing package of prospects is a bad idea because the fanbase would turn on the organization, or at a minimum you'd lose the opportunity to sell #34 jerseys which are a significant source of income. In a similar fashion, I think releasing Lynch could have some extraneous and hidden drawbacks that would make the team more hesitant to move on from his deal early.
Additionally, this front office has made a MAJOR emphasis on treating their guys right. They said re-signing the relatively expendable Kam Chancellor was their top priority, and when they signed the extension they even had a big ceremony with Kam to celebrate it. They paid way more than people expected to keep Bryant. They've been locking up their own players very aggressively and paying them at market value. Lynch is the ultimate PC/JS Seahawk, so he's much more likely to be treated with deference.
Is Lynch's security a good thing? That's debatable given how good his backups are. I suppose if Ricky Watters could lose his job to Shaun Alexander, Marshawn Lynch could lose his job to Christine Michael. Keep in mind though, Lynch isn't some mid-round RB who did well for himself. He's a former top prospect that was drafted soon after Adrian Peterson. I think for Seattle to move on from Lynch, it would have to be abundantly clear that his time here is done or that he's holding back the team in some manner.
One other thing to consider, Seattle gets no compensational pick consideration for players they release. So if they keep Lynch that extra year, they'd likely earn an extra draft pick from it, maybe a good one. When Seattle picks and chooses which guys stay and which go, I'm guessing they'll prefer to lose free agents over releasing players on active contracts, because players that leave in free agency could bring draft picks back the next year.