This is an easy one.
Mack: Has had trouble staying healthy. He can't handle being the workhorse alone for a whole season + playoffs. We drafted a back to keep Mack at 100%. He will get his carries still. We did nothing wrong by him.
Hooker: Signing his option would have meant paying him a top 10 S salary this season. He has not had the same effect on our defense as Earl Thomas, Harrison Smith, Jamal Adams, Kevin Byard, Devin Mccourty, Adrian Amos, Derwin James, Malcom Jenkins, Eddie Jackson, Tyrann Matheiu. I am sure there are other safeties I could list here but these names came to mind. Hooker just has not been a difference maker. Now, he has 1 season to show that he IS a top 10 safety and then the Colts can resign him. The Colts are just making him earn his pay... no harm done.
Brissett: We traded for him and gave him a shot as a starting QB. He didn't look like a guy that could lead us to a Superbowl and that is the goal right? So we brought in Rivers for a year AND kept Brissett on the payroll. He is on the roster to get another shot if Rivers (almost 40 years old) gets hurt. He is being paid like a starting QB.... we did no wrong by him either.