Knighton was also miscast in the 3-4, and didn't play that poorly in any case. He's effectively a poor man's Vince Wilfork, in that he's basically unblockable one-on-one in the run game and can create a push from the inside but will never be known as a strong pass rusher. He's an upgrade from Siliga and probably from Branch, as well.
Knight's big issue for his contract situation is that he's a nose tackle, and nose tackles are always undervalued because they don't put up big counting stats. There's absolutely no reason Ted Washington, the greatest nose tackle of all time, shouldn't be in the Hall of Fame, but he won't even sniff it.