(Not trying to be argumentative here, sorry if it sounds that way. It feels like this point is heavily missed)
Basically, for people like me, morality would dictate that it's wrong to arrest or imprison a person for doing something that is, at worst, a bit unhealthy. Still, just as many people split the other way and think it's immoral to promote "vice" at all. The balance may shift but it's there.
Contrast that with the economic argument and you'll have a hard time finding people who would choose revenue going to the black market over funding something useful or lowering the tax burden on those who don't buy cannabis.
We should be striving to bring overall tax rates for tobacco and marijuana up to match alcohol.
I'm really looking forward to seeing data on what happens to the black market for other drugs, with marijuana now easier to get legally than illegally.
source: https://en.wikipedia.org/wiki/European_Union_value_added_tax
I for one would love to see a comprehensive study on the cohesive "value" of both drugs in society and alter taxation accordingly. Alcohol has to cost the most for highway patrol and other social services. Decriminalizing marijuana has to have a substantial savings. Taxation is fair - sin taxes are completely optional and the state needs money. It sounds like the weed tax is too high and the alcohol tax is too low.
Policies like this are almost always better-off as economic rather than moral proposals.
It's hard to get people to agree to what's right, whereas it is distressingly easy to get people to agree to what's profitable.
Tourism revenue might see a boost though, but that may decrease over time if other states adopt legalization. It also may be offset by an increase immigration of drug users and emigration of non drug users (assuming you can accurately predict non drug users are more economically valuable than drug users)
Just looking at the topline number I think is pretty meaningless. I think also couching legalization in term of economics might be setting things up for failure and it might make better sense to say it's just philosophically the right thing to do(??)