it is very hard to deny that drugs can be debilitating to the point where a person cant or wont work.
Yes they can - but the question here is whether after legalization we'd be picking up the tab for many more of those exercising their rights to smoke - and the answer is that there's no evidence that we would.
And where the Libertarian argument falls apart — because, by practicing their right to do drugs, we all end up paying for their “right” in the form of welfare. So, no they are not “just harming themselves” but are a burden to the rest of us.