I don’t think they’ll go back to the 3-4, at least in a traditional sense when it was a straight up nose tackle and two 5-techniques. They just don’t have the two-gapping defensive ends. Deaderick is almost a fit, but he’s certainly nowhere close to what Richard Seymour was.
Once they traded Seymour in 2009 and then lost Ty Warren in 2010 they just couldn’t run it effectively. They tried to plug Jarvis Green in for Seymour in 2009 but was rough against the run.
Mayo is certainly a better fit in the 4-3, where he can play more in space. They were playing him at the weak inside linebacker position in the 3-4 and he tended to get swallowed up down in tight.
Here’s a couple articles that expands on this a little more: