A lot do. Luciano did 10 years of a 50 year sentence, got out early for helping the US during WWII and was instantly deported to Italy and continued to run things in the states through Frank Costello. Costello also did some smaller stints and always came back out as boss, mostly because Luciano wanted it that way though.
Sam Giancana did something like 4 or 5 years when he was coming up. He got out and eventually ran The Outfit in Chicago taking over from Tony Accardo. Shit, Frank Nitti also ran The Outfit after Capone and he did some time before that. Paul Ricca went to jail when he was running The Outfit and was paroled and most believe he shared power with Accardo after getting out.
Thing is, if they get 20-30 years and come out when they are 70 or 80 years old, yeah the don't do much after. But if they get out in their prime they almost always go back to crime.