Every service exists to offer a benefit (direct or indirect) to the taxpayer.
Why don't we let companies compete to offer these benefits, and let people pay for what they want?
A good way to explore it is: if government ceased to exist overnight...what would be the order in which you would be impacted by the lack of services, and what steps would people take to plug these gaps, considering the fact that we live in a technological age with the internet and nascent AI.
It's an interesting thought experiment, but it fundamentally misunderstands the reason for government to exist, which is not to turn a profit.
However there are lots of services which are in-fact inherent monopolies. Take electricity transmission or your ISP for example. They have a common set of infrastructure which is hard to share among many competing interests. Who gets to decide what should be upgraded, to what, and when? How does one divvy up costs infrastructure changes which did not benefit a significant set of participants because they are individually in the minority and unable to cooperate? If you prioritize minority participants then what about the majority participant? What about a super majority participant?
Lots of times cities will try to regulate these services tightly because past behavior has necessitated it. Though ISPs are perhaps a bad example here.
I read an article somewhere talking about a study where privatized healthcare resulted in increased profits but patient services suffered. Is profit above all else a good objective for government services?
Would all roads become toll roads? Do we just let the bridges crumble because there aren't enough people driving on them?
If you pay for your police and I don't, am I the one arrested even though you were the one who broke into my house?
Are companies that don't pay for the EPA allowed to pollute to their heart's content?
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The entire point of a government is that it can make long-term (generation spanning) decisions that might cost money and would never return a profit.
In practice it leads to underfunding of public schools because the people with money are the people with political power. If their kids went to purely taxpayer-funded schools they would be higher quality and everyone would be getting a better education. (I know in the US things are a bit different, and there are are high quality public schools).
The argument then has two sides. One side is the freedom argument: parents should be able to do what they want within reasonable bounds. Seems valid and clear-cut. But then the other side points to the long-term consequences of having a bad education system. Lower economic growth, more populism if you have a stupid population with poor media literacy skills, and all the associated unpredictable unintended consequences that don't fit nicely into the whole "freedom" discussion. What was that freedom worth if I now have to live in a dictatorship because of all the accumulated unintended side effects that boiled over?
If you are one of the few, who are getting a good cut of the every-growing profits, then the latter is nice. More so if you're one of the very few, who are too rich to care how much the "best care" often costs in America.
With pay-as-you-go if you don't have the funds, you simply bear with the circumstances until you can afford it. For example, if my electricity is cut, I am forced to use a duvet with two blankets on top to stay warm when if I had the funds, I don't need to, since the heat is paid for.
Same with Internet. I sometimes have no Internet for the day, but when funds arrive I can get back online. But those moments are teachable: I simply read a book, or clean the house when there's no Internet. Subscriptions are a marketer's dream. They want you hooked. Fuck that.
Try getting a bridge built to the other side of the river where no one lives to expand the city.
That's what education vouchers do, and many Republicans do support them. For national defense there is a free rider problem -- the military would protect your country regardless of whether you individually "subscribed". The general answer to your question is that the federal government largely exists to transfer money to the poor and the elderly. Big taxpayers are not paying primarily for services but for such transfers.