If they did so without paying taxes they'd be violating the tax laws. Why would they risk it, since the profits to be had would no longer be inflated by anti-pot laws?
The government's never going to starve for lack of alcohol taxes, but with pot I think if it became legal it would be much harder for the government to capture a large percentage of the market.
There might be somewhat more home growing than home brewing, but I know of no reason to predict a drastic difference.
In fact, I think if they were to legalize pot it may actually cut into sales of eithor booze and/or some prescription drugs.
Another argument in favor of legalizing pot; I'd rather deal with a stoner than a drunk any day of the week, and many law enforcement officers agree.
People who make money babysitting, mowing lawns, housecleaning, etc. technically should be paying taxes too, but how many do you suppose actually are?