My understanding is that the Saints set prototypes based on the average height/weight/speed/quickness, etc. of the average players at each position and they try to stay close to those numbers in players they draft. They will go outside of those prototypes, but the guy has to have a special quality that makes him an exception to the rule that players that don't meet those numbers don't make it at that position.
Anway, my further understanding is that the scouts just scout guys and issue reports on each player. Then at some point before the draft Loomis, Ireland, the HC, and now Mueller and maybe Parenton, stack the draft board based on how those guys fit into the schemes the team runs. Then when the draft comes, in the early round Loomis, the HC, and Ireland try to reach a consensus on each pick which is has almost always happened. If they can't Loomis makes the final decision. But in the middle to later round they rely heavily on Ireland to pick the players.
If that is how it works, it could be an issue at any level from the scouts to Ireland, to the HC, and to Loomis. The HC is changing but it has changed before and the results are similar so it seems likely we have issues either with the Scouts, Ireland or Loomis. Or all three. But since Loomis defers to his HC on players in the vast majority of cases, I suspect he is not the issue on this one specific thing (yes he has other issues and isn't likely good at player evaluation). So I think it comes down to Ireland or the scouts and Ireland picks the scouts.
That being said, you are right that a lot of it could be a lack of developing good players by the coaching staff.