It is far too soon to say that Johnny Manziel has "sealed his demise with his antics". There are plenty of players with off-the-field problems and terribly personalities how find spots on NFL rosters. As long as Manziel is able to perform on-par with or better than other Quarterbacks in the league, he will find a team that will look past his off-the-field antics.
Johnny Manziel reached what some may think is his peak performance in college. Due to his age at the time, some of the antics he engaged in while in the spotlight may have brought unwarranted attention and negative views to his football career. I believe he still has some to change his ways and can have an excellent career moving forward.
I think he is still very young and has done some dumb things and probably parties a little too hard, but I still think he has time to turn it around. If he has a good rookie season then I think players and fans alike will forgive his misdeeds. He also needs to realize that there is no such thing as privacy for him anymore.
Johnny Manziel is still very popular, and at the moment he's riding on that. While he's popular, and while he continues to play well, people will give him a pass on the wild parties and his lifestyle. As soon as he begins to play less well, it's likely that the fans will suddenly remember what else they've seen him do.