It’s not just the pot they want to legalize.
It’s the other hard drugs like Cocaine, Heroine and Meth that are next in line. ***
So even if this were true and some states decided to legalize those; which I think very very few would for a long time, because those drugs are used way less than pot; then you would have a situation where the drug users would simply live in those places where the drugs were legal and you could live in a place where they are illegal. Problem solved for both sides. Cocaine used to be legal too, btw.
Availability of drugs would not make drug users go live there. That's just ridiculous.
And just because cocaine was "legal" at one time doesn't mean everyone used it. In fact very few people even knew what cocaine was.
Opium was once legal as well for a time in the UK. However, its use as a pharmaceutical panacea and exotic recreational drug became epidemic throughout all of British society.
Perhaps you should read up on how opium nearly destroyed entire nations before you daydream about drugs being "legalized". Legalized pot would be a repeat of what legal opium was in India and the UK, since both make people extremely lazy.