I hear you - if I no longer paid my tax now, it wouldn't make a difference to the ATO financially. However, if all middle-income taxpayers in my demographic who pay between around $30,000 and $80,000 in tax were to stop paying the ATO, they would feel that.
I forget the tax brackets decades later, but $90x2-3 days a week was getting taxed enough that I was losing about two hours per shift on tax. I remember thinking of it in hours at work. The first two hours paid Mr. tax man, the remaining six were for me. I remember that I was making $7 per hour. My annual tax came out at less than $2k, before deductions etc. If all the kids in the same boat as me didn't have to pay tax, I doubt it'd make much difference to the ATO.
In Australia at least, those cards are everywhere in populated areas. Supermarkets and department stores pretty-much all stock them. I'd say that over 90% of Australians live within 3km of a store that sells Steam cards and takes cash. Most of us even closer than that.