ELI5 why shouldn't a government aim for 0 debt.

It really doesn't matter. Things that don't give a direct income do need to be built. We have 3 options:

  1. Save up and build it. But this assumes you generate more in taxes than you need which puts pressure to cut taxes. But even if you do save up, you've only saved and then spent, rather than spend and then pay the loan. You might say the loan costs more because of interest but that's not exactly true. Money lying in your bank account waiting to be spent has a cost attached to it, the minimum return you could generate if you invested it which means the amount of time your money spends sitting, waiting to be spent has a % of return you're giving up attached as the cost.
  2. Let private businesses build it. You could but then you have haphazard building because private businesses build for their specifications and don't care what the guy next door builds. So you may get a stretch of road that is 4 lanes, then 1 lane, asphalt in some parts, concrete in others and mud elsewhere. And since businesses aren't charities they won't build roads that anyone can use for free. There will be a toll to pay which then is no different than you paying the interest on debt through taxes vs paying directly. The big downside is that businesses don't build up in places that are falling apart since poor people don't have money to spend. So areas without roads tend to remain that way for longer, though not interminably.
  3. Take a loan build it and pay the interest. Yeah you may need to raise some taxes to pay the debt off, but it's a different way to spend the same amount.

this is one view and there are others. Each society decides what it wants and what it can have depending on the hand it's been dealt. For example, Saudi Arabia had no income taxes or debt for a really long time since oil revenue was rolling in. That is slowly going away. Resource poor places on the other hand have always needed someway to make money for these things. In the past it was by owning the mean of production (state run banks, companies, post services). As that goes away some other means are needed.

NB: there is of course such a thing as too much debt or unnecessary debt.

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