If Patricia already has one foot out the door as you suggest, that is even a better reason for the Lions to draft Tua. That way he will be fully recovered from his hip surgery by the time the new head coach takes over in 2021.
Stafford is a good QB when he is healthy but he just hasn’t been able to take the Lions offense to the next level. It might be time for the Lions to do what the Dolphins did with Tannehill and just start over at the QB position. On a more talented roster Stafford would likely have similar success to what Tannehill had with the Titans.
If Tua is truly a generational type QB he is said to be, I have no idea how the Giants, Lions, or Redskins wouldn’t draft him. Because if he is able to stay healthy in the NFL, his skills are much better than any of the starting QB’s on any of there teams.