Because he has played character Don Draper in the hit show 'Mad Men' for such a long time, there is a very good chance that Jon Hamm will lose much of his popularity when the show officially ends. He has played the role for so long, and viewers will not be able to see him the same way.
As many actors and actresses before him who have made their explosion onto the Hollywood scene in a television program have experienced, Jon Hamm's popularity is likely to begin to diminish at the conclusion of Mad Men. Often times, these actors are pigeonholed in one role, that of their first major character. Fans of the show will also struggle to see him in any other role outside of Mad Men.
It will be a dip in an acting career but if anything I would assume that it might make him more attractive as he will now be able to focus on new projects and I would think that people will want to be the first to get him to play a new character since his popularity is high right now.
Jon Hamm has acted before Mad Men and can act after the show ends. He was never as popular and famous before Mad Men, but with hard work he can use this new popularity to his benefit. He really just needs to find a role that is right for him. Some actors are successful transitioning from TV to movies and that might work for Jon.