Probably many would find something else, and that's a problem - but surely it can't be used as justification for not changing the policies. Legalization should happen gradually and with some kind of amnesty outreach program. But even if not: Crime like robbery would likely spike, but unless gangs quickly find a replacement product that's in similarly high demand (if there was one, wouldn't they already be serving it?) I imagine cutting off their easiest source of income would fairly quickly reduce their power to attract new members, purchase weapons, etc.

And even more quickly, since it takes a LOT more money, reduce their ability to bribe cops and judges.

Actually, crime went up a lot during the Prohibition period. Why would one illegal good differ so much from another?

EDIT: removed fragment: "This is extremely well known". I disagree with the parent, but let's keep the debate as civil as possible. My apologies.

I think you're underestimating the attraction of drug dealing as an occupation. Its profit vs. risk ratio is a lot higher than kidnapping.