We've ranked 1st, 7th, and 8th offense since Lamar has been the starter fulltime. You can't replace him and expect the same or even similar results. We'd slide back into the days of bottom third offense especially with the surrounding cast we've assembled around Lamar.
I am wary of handing him a guaranteed contract to pay him amongst the top QBs in the NFL. I just don't think you can say we could replace him without stepping back significantly on offense.
There would be a step back. It would also come with plenty of financial flexibility to add weapons, OL help, and defense.
Look around the league at who is relatively good without a star QB.
Seattle
NYJ (maybe the worst QB group in the NFL outside of Houston)
NE
Minnesota
Tennessee
NYG
Washington
49ers
Now look at the teams that have "good" QBs who are below .500
Green Bay
Tampa
Arizona
LAR
I'm not saying that having a good QB isn't great, but it isn't a guarantee of a good season nor is not having one a death knell to playoff aspirations.