According to the SF Chronicle, about 72 million US households (40% of the population) paid no federal income tax in 2022. I don't have exact numbers but I doubt anyone would dispute that public benefits flow disproportionately to those 72 million households for obvious reasons. So the cause and effect of being a citizen and paying taxes is very tenuous.
> Where exactly is the unsustainable part of this?
If it's unsustainable, then someone should propose a constitutional amendment to fix it.