2. Mariota's exit was clearly quitting on the team
From Mariota's perspective, it is completely understandable to have a tough time with being benched and want to spend time with your family that is celebrating a new addition. All of this is completely defensible and makes sense if you're looking at it from the veteran's perspective.
However, watching Mariota quit on the team and knowing a rookie quarterback was going into the lineup hammered home that Marcus did quit on Atlanta. There were four weeks left on the season and taking the time to be there for the rookie and his teammates would have been ideal.
Arthur Smith protected Mariota and put him in the lineup even when it clearly wasn't working deep into the season. The head coach fought for the veteran and this was repaid by Mariota quitting on the team when having the veteran as a backup would have been ideal.
Perhaps it would have even opened the door to Mariota sticking around for another season on a lesser deal to help Ridder. However, the way things ended is far from ideal and Marcus deserves some heat for simply leaving when clearly being benched was deserved.