Not that I couldn't possibly see Letang performing until he's 40 (dude is a gym rat) but wasn't Gonchar extremely well... bad... by the time he was 40?
Yeah he was, he was a training camp invite in 2015 to the Penguins at age 41 and he was atrocious.
Here's how Gonchar performed in his age 35-38 seasons:
Age 35: 50 points in 62 games with a -0.55% xGF%Rel with Pittsburgh in top pair usage
Age 36: 27 points in 67 games with a -0.55% xGF%Rel with Ottawa in top pair usage
Age 37: 37 points in 74 games with a -2.78% xGF%Rel with Ottawa in top pair usage
Age 38: 27 points in 45 games with a -1.83% xGF%Rel with Ottawa in top pair usage
I think Gonchar really fell off a cliff after leaving Ottawa. He had a bad year of production at age 36, but the rest seems pretty solid overall. After leaving Ottawa, he went to Dallas where he regressed to a bottom pair guy. His results weren't bad, but he was only playing bottom pair D usage.
This is generally what I expect out of Letang, outside of the bad age 36 season with production. He'll stick around at a good level for about 4 years and then he'll really fall off a cliff at age 39.