Vendors won’t see RoI on their ad spend and will stop buying. Simples.
Governments see reduction in tax income and GDP and repeal taxes. In UK: Signal "mansion" tax on properties valued above X price to collect recurring Y pounds total tax. Market adjusts valuations based on probability of tax. Number of houses still worth at least X now diminishes. Now cannot collect recurring Y pounds. "Mansion" tax delayed.
You have this complex system that has reached some sort of relative equilibrium based on say a set S of ten sorts of tax rates, along with a set F of factors (size millions), with the government's tax revenue R being one of those outputs. Then some guy in the government called G signals to the government and public that he can increase R by X by fiddling with a member of S, or maybe adding a member to S (of size say ten).
Is G stupid, or does he just lean towards retaining the public's affection, his relatively low salary, potential under the table payments and whatever networking opportunities his job provides? I lean towards the latter.
Vendors now cannot get X pounds with Y pounds advertising outlay to make Z pounds per unit of wares. To continue making Z money per unit of wares, with previous S pounds price charged to consumers per unit, add significantly more than reduction in Y advertising per unit to S to offset reduced "brand" "awareness".