Good point, although there may also be administrative overhead in just keeping these ancient bonds on the books, so the total cost may be higher than 4%.
As far as future interest rates, there is an argument that as a civilization matures, interest rates trend toward zero. The more mature the economy, the lower the perceived risk (on average), and so the lower the rates. Apparently in the later stages of the Roman Empire for instance, interest rates were very low. So it isn't a foregone conclusion that interest rates will ever return to the "normal" levels of the 20th century, at least until a new global paradigm comes along. (Nor that they won't, of course.)
> there is an argument that as a civilization matures, interest rates trend toward zero.
I'm inherently suspicious of any argument that is supposed to apply to "civilizations". Not that it doesn't matter, but there are usually much more simple explanations to be found elsewhere. For example, whatever we know about money in the Roman empire, we know far more about money in Britain, US, France, Germany, etc., in the last several centuries.
> So it isn't a foregone conclusion that interest rates will ever return to the "normal" levels of the 20th century, at least until a new global paradigm comes along.
The way I understand interest rates work, from conventional macroeconomics, is that some people have money to spend and invest, and some people want to borrow money and spend it on something that will make them more money. So when you look at the interest rates, you aren't really looking at the riskiness of a modern government (US is definitely risk-free, UK also probably), you are mostly looking at the "price" of the people's savings.
When lots of people want to save, interest rates --- the price others are going to pay to borrow --- will go down to balance the supply and demand of savings. So one way of looking at low interest rates is to see this as a situation where there is an excess of desired savings (i.e. the interest rate that would truly balance savings and investment is below the current rate, which is already almost zero in real terms). This explanation is incomplete (you need to introduce another constraint, and do more macroeconomics), but it's more conventional, and I think also more robust than most theories of civilization.
As far as future interest rates, there is an argument that as a civilization matures, interest rates trend toward zero. The more mature the economy, the lower the perceived risk (on average), and so the lower the rates. Apparently in the later stages of the Roman Empire for instance, interest rates were very low. So it isn't a foregone conclusion that interest rates will ever return to the "normal" levels of the 20th century, at least until a new global paradigm comes along. (Nor that they won't, of course.)