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.