Why aren't pensions sustainable? Honest question.

At least in the US, there seems to be more than enough money to keep several branches going. The military just got $20 billion extra this year. Foreign aid is around $45 billion a year.

It's not that there isn't enough tax money.

They inevitably fall into the trap of promising that benefits will be paid for in the future.

At a national level that is probably okay, at a municipal level it means that people that leave a city get services that they don't pay for, and future residents pay for services they didn't benefit from. It's a terrible alignment.

You are describing pensions. That's how pensions work, and have been working for decades now.

Would creating a national pension system work better?

No, I'm making a particular point about public pensions. They should be fully funded during the vesting period or not exist, except maybe at the national level.

Note the "paid for" in "paid for in the future".

Sustainable implies “don’t need to print money forever”.