Statistically, and in my experience, drug dealing is not a violent crime. They're not going to start knocking over convenience stores. Some of them might scrape together enough capital to start a retail store. Prices and profits are going to take a long time to fall, so most of them will just keep doing what they're doing, except with a little less risk, and they'll be able to keep their cash in a bank, and theoretically pay taxes on it. If things reach the point where it's not really profitable any more, then those people will have to get a real job.
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.