I mean he always played off ball LB in training camps and has always been listed as a LB or SLB. They have never listed him as an Edge or DE. Maybe the plan wasn't for him to replace Demario, but the plan clearly was for him to play off ball LB regardless of if it was MLB, SLB, or WLB. And he played SLB in games because Demario and Werner were the two starting LBs so he only got on the field when we played 3 LBs.
DA clearly decided by the end that he didn't think he was going to be able to play off ball LB at a high enough level to start over Werner or Demario, and that might have been a mistake depending on how you rate Werner, but it's not like the Saints didn't offer him a contract. They apparently offered more than Philly but he took the Philly offer and bet on himself to win a starting job there and then get a bigger contract.
It sucks but it happens. But I guess it comes down to do you view this, along with Ellis playing well in Atlanta, as proof that the Saints staff is good at developing LBs or as evidence that they are dumb because they keep letting good LBs leave? I think the answer depends on how much you dislike DA and how down you are on the organization in general.