Narcotics Police Squads are created out of EXISTING police officers... nobody gets hired on a department as a narcotics squad member.
The DEA would still have plenty of work to do dealing with already legal via prescription drugs, and ensuring that there doesn't remain an underground drug trade network avoiding the taxation if said drugs became legal.
Prosecutors would still exist because there are still crimes to prosecute.
Defense attorneys would still exist because Ive never heard of a defense attorney who only defends drug charges. (Specialize yes, only do? no)
Prison Guards would still exist because there would still be inmates.
Wardens would still exist because there would still be prisons.
If anything, many of those involved in organized crime that are serving time under drug charges would be there for something else by now. Many people are attracted to crime, just like many underage kids love drinking booze until they turn 21... then it just ain't all that special anymore.