I think it’s a combination of two main factors:
1. Cousins is coming off of an Achilles injury and the Falcons don’t really know how Cousins will perform or hold up. Drafting Penix is an insurance policy on their investment in Cousins. QB play is vital to a team’s success in the NFL, and the Falcons had poor QB play the last couple of years.
2. The Falcons may lose their first round pick next year due to the alleged tampering with Cousins. Cousins is getting older and the Falcons want to plan for their future.
You could argue that the Falcons think they can easily win the division with good QB play, and that they don’t think they are as bad defensively as has been stated on ESPN. Time will tell if they made the right choice, but I think it was a bad decision when they have no pass rush.