We need impact players at LB. We don't have any. Bertrand is probably functional because he calls the defense, Liufau runs around with his hair on fire, rarely making key plays and Kiser plays more controlled than Liufau but doesn't really achieve any more. All of them are replaceable.
For this defense to be better it needs better players at LB, probably more than any other position. And if you look at the depth chart, it is the most logical place to move on to better players.