Can you name any evidence supporting the necessity of, to pick a moderately troublesome example off the top of my head, copyright?

You seem to be expecting a much higher standard of evidence than I had in mind. Perhaps necessity was too strong of a word. Utility? Benefit? Something like that.

All I ask is that laws have 1) a clearly defined goal of solving a problem that society wants to solve, and 2) empirical evidence (gathered after the fact, if needed) that they are doing what they were intended to do with acceptable side-effects. Marijuana criminalization seems to badly fail at least the latter, and the former depending on what problem you think it's solving.

The examples you use both have straightforward utility (compensating positive externalities, reducing death rates), and mixed evidence of effectiveness (lots of art created but copyright terms of infinity minus epsilon inhibit building shared culture, misuse of legal firearms suggests more powerful weapons would also get misused for greater potential damage but firearm crime correlates poorly to ownership rates).

Prohibiting by default strikes me as untenable on practical grounds, as well as being morally dubious in the extreme. As an aside, however, I actually would support abolishing intellectual property as weakly superior to the current scheme, but I doubt either is optimal.