I've known areas of my town get dramatically safer after some big drug busts. The drug trade has significant externalities.
I got it! We'll outlaw poverty!
The idea that they will turn to some other criminal pursuit...yeesh. It's like you've never even met any dealers. Imagine that tomorrow, the government made your job illegal. Would you be like, "Well, that does it. I guess I'd better start brushing up on my raping, murdering, and insider trading," or would you maybe not go all-out breaking whatever laws you could? It may shock you to realize that because the hypothetical dealer is breaking the law in one respect, he is best served by not breaking the law in any other respect, in order to not draw attention to himself.
Lastly, we don't have to speculate what will happen, there are countries which have decriminalized drug use -- US states too. Marijuana has been decriminalized in Alaska for almost 40 years, although no one seems to know this. The sky has stubbornly refused to fall.
Violence may not be required however in cases where everyone is already scared witless of them. Drug crime and other crime go hand in hand.
If you legalize drugs completely , then serious businesses run by people with MBAs , Venture capital, marketing teams, legal departments etc come into the game. Rather than street level violence, battles are not fought in patent courts and with government lobbying.
Those who are currently dealing drugs don't know how to play this game and will lose.
They can't go and get a straight job because they have no provable work history, probably a criminal record and few contacts outside of the "underworld". They also have become accustomed to a lifestyle of nice cars and (possibly multiple) high maintenance girlfriends, so McDonalds isn't going to cut it.
So they will look for other lucrative opportunities where the MBAs don't want to compete with them (because it is illegal).
"Drug crime and other crime go hand in hand." [Citation Needed]
"Those who are currently dealing drugs don't know how to play this game and will lose." And then get a real job.
"They can't go and get a straight job because they have no provable work history, probably a criminal record and few contacts outside of the "underworld". They also have become accustomed to a lifestyle of nice cars and (possibly multiple) high maintenance girlfriends, so McDonalds isn't going to cut it." You have no idea what you're talking about. The margins are not that high.
Your image of drug dealers applies to an extremely small minority. You must only know what these people are like from TV.
You claimed that drug dealing is a non violent activity. I showed an example of where that is demonstrably not the case. Roughly 65% of the coke in the US comes through mexico where it is causing serious violence.
In my home country (UK) we also have problems with violence and intimidation from drug gangs in many deprived areas.