It seems to me that if you decriminalize drug usage and do away with the drug war, drug kingpins aren't just going to sit idly by and let their kingdoms go to pot. So how can we keep the peace and protect people from the violence of gang warfare and drug wars after drugs are decriminalized?
When alcohol Prohibition ended, the gang wars ended, too. The Mafia focused on other illegal activities when re-legalization took the profit out of booze. Crime decreased dramatically and so did the need for police.
When we end drug Prohibition, we can expect the pushers to fade quietly away also. When we re-legalize drugs, we'll take away the black market profit that attracts the criminal element. Drug wars will stop because they won't be worth winning. People won't need to steal to support their habit because legalization lowers prices. Just as with alcohol Prohibition, crime will decrease dramatically and so will the need for police.