I have never heard of an argument that linked drugs and welfare.
First, drug users now get welfare.
Second, legal drugs should have the price go down since most costs are associated with reacting to law enforcement.
Third, legal drugs should kill many illegal drug users, decreasing welfare costs.
If you haven’t had the welfare rag waved at you here on drug legalization then you haven’t been in some of the scraps I’ve been in. And I’ve pointed that if Target and Walmart were competing to deliver the highest quality meth for the lowest cost, the meth problem would fix itself toot-sweet from all the over doses. That argument seems to work since they have a hard time being against dead drug abusers.