You have a point, but KJ was released before the switch to the 3-4. They also thought Bobby Wagner was over the hill at age 31. I'm just saying that I don't think John and Pete will pay money for an aging LB.
In addition, although I think Lavonte David is great, PFF projects that he's going to cost $22 million over 2 years. I don't see how that fits into our budget. I think Germaine Pratt compares favorably to David, and the guy plays with tenacity and heart.
They were already running a lot of 3-4/Bear fronts while KJ was playing - look at games his last year and see how often he was playing up on the line of scrimmage. They didn't want to playing him at off-the-ball LB bc they didn't want to stunt Brooks' development. And the year they moved on from KJ they were definitely moving towards a more decisive shift to the 3-4 like we see now with Nwosu, Taylor, and Mafe playing up on the line of scrimmage and doing a lot of pass rushing.
Point is I don't think age was the main reasoning with KJ, and as was pointed out, it wasn't really with Bobby either - it was his contract.
That said, if David costs as much as PFF is projecting as you point out, then that's probably off the table. Draft might be the best bet at this position, and at least one of the top LBs should be there at the top of the 2nd round, if not lower.