If you want to have realistic expectations for Arik Armstead, set them low. Sea-level low.
Because even though he recorded 10 sacks in 2019, he never has recorded more than 3.5 sacks in any other season of his life dating back to college.
Here are his sack totals by season:
Armstead simply is not a pass rusher. He's a good run defender who gets paid like a pass-rushing specialist. And when he rushes from the interior and is flanked by three defensive linemen who are better than him, he's a net positive on the 49ers defense.
But last season, Armstead was the 49ers' best healthy defensive lineman after DeForest Buckner got traded and Nick Bosa and Dee Ford got injured. So the 49ers moved Armstead to defensive end full time, and he was a net negative. He couldn't generate pressure on the opposing quarterback or keep him in the pocket. He repeatedly rushed wildly from the edge and allowed the quarterback to scramble to his right and fire passes down the field. The 49ers defense would have been better off had Armstead simply stood still on the field as opposed to trying to rush the quarterback.
But this season, Armstead should move back to the interior when he rushes the quarterback, because Bosa should be healthy, plus the 49ers signed defensive end Samson Ebukam. And as an interior rusher, if Armstead can record just four sacks, he'll be a net positive once again.
That's the most you can reasonably expect from him.