Pay him.
I don't think he's a risk to regress after an extension like a Miles Austin or Tyrone Crawford (lacking pure talent, not elite, only 1-2 years of top-level play), Bryant (head case, unwilling or unable to adjust to effects of injury), Marion Barber (head case, short shelf life position) and so forth. Extending him now is a lot like extending a prime Witten, you have a guy with a long track record of elite play, a great work ethic, his game isn't reliant on explosive athleticism so he'd be more durable coming back from an injury and no red flags on or off the field.
There is significantly less risk in extending him compared to other candidates like Lawrence (health, only one year of elite play so far) or Irving (head case, never had a full season of elite play).
The only real issue is the relative value of his position but that is more of a theoretical/philosophical thing, this team is more reliant on the OL than any other in the league so I think that argument goes out the window for this particular organization.