S42,
On a recent online town hall for unassigned Delta pilots, Delta said they want to have all unassigned pilots back...and through training...by early August. That's ~1700 pilots. The Puzzle Palace has a big job ahead of them to accomplish this...if they can. Even with farming out sim time it's still a tough order because there are only so many sim instructors and check airmen.
So Delta apparently feels it's necessary to get all the pilots back on board.
And what lessons were learned ? That no one knows for sure what's going on.