Don't be silly, no system which relies on something that is inherently worthless i.e.debt, can last forever.
Debt is not inherently worthless. It can be productive and generate a return. Easy example, if I loan you my lawnmower, and you cut the grass with it, you have been productive. From the time you pick up the lawnmower to the time you return it, you are in my debt -- (1) lawnmower.
Usury, by its very definition, is inherently worthless.
Debt in the abstract is worthless. I'm also not sure I would described the situation above as 'debt' rather I would say the person has an obligation to return the lawnmower.
Modify the scenario slightly. Joe borrows a $2,000 mower from Dan, then proceeds to mow the lawns of 20 suburban homes over a week-long period. Each homeowner nets Joe $20 profit so he collects $400 then returns the mower to Dan along with a $100 check to cover wear and tear and compensate for Dan's inability to use the mower for a week.
Effectively, Joe borrowed $2,000 over a week period, was productive and paid back the principle plus interest yet came out $300 ahead. This is how loans can be used to benefit society.
I'm not really talking about that, what I am saying is that the sale and re-sale of debt that goes on today in the financial markets isn't sustainable and neither is the addiction to debt of average people and governments. I'm perfectly happy to admit that lending things to others can be beneficial and even that loans are sometimes necessary but when the entire economy runs on something which in and of itself is worthless its not particularly healthy.
I think one has to define sustainable in terms on human lifetimes or generations.
No money system is without problems. Many have collapsed in history usually to be replaced by one with different features so that the same problems don't arise, ( but different problems do).
Ours is not perfect, it will surely collapse one day, but when and how I am sure that nobody on this board knows.
If the money system lasts for the rest of my life, with a few tweaks, then as far as I am concerned it was sustainable. I have neither the power nor the responsibility to set economic policy. The best I can do is make a healthy living in the economy I am in.
Frankly, I haven't found it particularly difficult to do that. There are tons of opportunities in western countries and tons of money sloshing around in businesses. One only needs to find a problem to solve or a way of making them more profit, stopping them getting fired or going to jail and businesses will spend money doing that.
If I hadn't been so lazy and enjoyed so many hobbies and distractions I am pretty sure I could have made squillions. But I've never seen the point of sacrificing my life for that. How many sirloin steaks can you eat?
If you cannot make a decent living in the economy today, in western nations, then I fail to see how you would have made one in the past with trade tariffs, local monopolies, inefficiencies, infectious diseases, world wars and religious persecutions to deal with.