1. Kirk's contract demands he starts through the 2025 season
This is the biggest factor in why the Faclons simply won't consider starting Penix this season or next year. The only path Penix has into the lineup is via injury or if Cousins is so awful next season they are willing to sit him down and pay his salary while starting Penix. Those are the paths that the rookie quarterback has into the starting lineup.
With how the Falcons built Kirk's contract they can escape in year three but not before. Cutting or trading away Cousins before this point results in a dead cap hit that cannot be accepted. It would put your roster in an awful position and give you zero room to build around Penix.
Even if Cousins were struggling consistently at the level we watched at the last two weeks of the season it is hard to believe the Falcons would give up on the veteran. Kirk's deal is a four-year deal that is really going to only be good for the first two seasons. After this, if Cousins isn't playing at a high level or the team is ready to move to Penix you can survive the third-year dead cap hit.