Evidence from Colorado ten years after legalising it suggests that whilst use went up, behavioural and legal problems associated with it did not. Interestingly there are studies that show in areas of the US where cannabis is legal, opioid usage and deaths have dropped.I'd say there's more likely a sudden drop in use.
That said, we'd have to do it properly. I know there's a market for illegal cannabis use in the USA because the medical stuff isn't very good.