You're way off. Romo wasn't even remotely himself when he came back to play. It's true that the team wasn't either. He came back to a team that needed rescuing. As the quick-witted improvising Romo that we've come to know, he may have had a shot at it, a remote one, but a shot. He came back a shell of himself in a desperate attempt to salvage the season. He's had an offseason to heal up now, and, hence, there's a good chance he'll be back to his old self. I'm not drinking Jerry's Kool-Aid that says he can play 5 more years. IMO that's nonsense, but I think he can not only play, but be excellent this season.
Btw, Cousins is not the only Washington QB to earn the starting job in recent history. RG3 certainly earned it in his first season with the team. He had a great year and lead the team to the playoffs. While that's what he was drafted to do, the team would not have stuck with him if he had been awful. Remember, San Diego pulled Ryan Leaf when he underperformed, despite having traded to move up and take him. RG3 appeared to have what it takes, then for whatever reason he was unable to keep his quality of play as high in subsequent seasons.