The super late coaching hire is probably why the Saints felt they had to poach LSU because all of the good DLine coaches are taken. It’s also why we’re hiring so many college coaches in general, imo.
Underhill has been saying that the Saints liked some LSU coaches since Moore was hired so this has likely been in the works for quite awhile. They probably had to wait until they hired Staley before they could complete anything.
As far as why they are hiring college guys, I think it's a realization that they are about to get really young on defense and they will need coaches used to developing young players. Some of it is likely a lack of options for the better NFL position coaches, but I also think it really is an intentional effort to get better teachers and guys that can develop talent. Looking back at this team and the guys who went on to star for other teams or guys that just never developed here over the last 5 or so years, it looks like the team had gotten away from or just wasn't very good at developing the young talent they had acquired. So, maybe it's been about lack of ability to develop guys as much more than bad drafting?