Budgets are fortunately generally not this inflexible.

A drop in alcohol sales would cause the budget to be reworked. If they decide to cut the education budget in response, that's fine, but they would not simply throw up their hands and say "oh well, no books for students".

If you worked two jobs, and deposited one paycheck into a bank account that you labeled "food" and the second into a different bank account labeled "rent, savings, and everything else", and you lost the job that got deposited into the "food" account, would you starve?

The point is there are perverse incentives. Societies really want lower alcohol consumption. But at the same time politicians know if it goes down they'll have to come up with the money via a different, maybe less popular tax.