Mahomes (good team but not contenders when he got there), Burrow, Allen, Lamar, Russ, and Hurts all had their teams as top teams before signing the big second contract. And Purdy can be added to that.
And even if teams were good before most of those teams realized that their QB put a ceiling on the team and made the change which just backs up my point. Don't double down on an okay to good QB because it limits the ability to put pieces around you and puts a ceiling on the teams potential versus going for a younger QB and using that extra money to build depth.