His 5v5 points per 60 minutes rate has been fairly consistent: 1.4 his rookie year, 1.3 in 2017-18, 2.1 during the breakout season, 1.5 in 2019-20, then 2.0 in 2021. Three years producing like a low end second liner, two years at more of a high end second line level. The fluctuating power play production is what makes his overall point totals look a bit all over the place.
Labanc is strictly a complementary winger but you're wrong about him needing Thornton or anyone specific in order to produce. He just needs to play with good players in general to be productive, he's not going to carry a third line with a couple of nobodies. He's not as bad defensively as people make him out to be either.
I wouldn't want the Sharks to go out and acquire Labanc but since they already have him and are basically stuck with him at least through next year's trade deadline due to his contract, I think he's fine in a second line role on a bad team and far down the list of reasons they are a bad team.