Somebody will take him, he'll get his [bleep] together and turn into a decent QB. Not many remember, but Brett Favre started his career in Atlanta. He was let go for similar reasons after 1-2 years. No internet back then so the reasons why were kept quiet. He grew up and turned into a HOF Quarterback.
He will get his 2nd chance. I think he'll either turn out to be one of the great ones, or he'll burn out in another year.
Regarding Brett Favre, his contract was traded to Green Bay on 02/10/92 for a first round pick, #19, in the 1992 NFL draft. Atlanta used the pick to select running back Tony Smith. Smith ran for a total of 329 yards during his entire career.
Regarding Johnny Manziel, I think that Jerry Jones is smart enough to distance his team from the train wreck that is all Manziel has been since the day Cleveland drafted him. Jones would be better off signing RG3 to back up Tony Romo for the rest of Romo's career. In a year to two, RG3 might learn how to be a functional pocket passer.