First of all, you are confusing currency and money. Most money (broad money supply of Australian dollars) exists as records in a bank. Only a small percentage of this is in circulation as physical pieces of currency (notes, coins) at any one time.
1. Money is created by banks. Banks have been given the power by the government to monetise debt.
Say you walk into a bank and take out a mortgage for 500,000. The bank takes your promise to pay and then types the numbers into their computer. They type in that you owe them 500k and they create the 500k credit out of thin air.
The Federal reserve in each country sets the reserve requirements (typically ~10%) so if they have $100 on deposit, they have to keep 10% so can only lend out $90. But that 90 can be deposited and then 90% of that ($81) can be lent out etc.
But if the banks lend out 90% of your deposit, how come you can take it out again without them calling in the loan? Because only a small number of people try and take their money out of a bank at once. If more than a small number did, the bank would collapse. This is called a run on the bank. One of the other purposes of the federal reserve is to inject money into banks that have runs to prevent banking collapses.
In effect money IS debt. The money in your bank account or you use to buy groceries was created when someone took out a loan or a mortgage.
Eventually when the loan is repaid to the bank, the money is destroyed again.
2. If everyone paid off their debts, there would be NO money as money = debt. If you have 1 million dollars in your safe, then that means that someone hasn't paid off all of their debts.
Gold is not a promise to pay anyone, nor are other physical goods. I was not talking about things of value, but MONEY as in legal Australian tender.
3. When you get a loan, you have to pay back interest and principle. But when you take out a loan, you only create the initial principle.
Thus, borrowers have to pay back more money than exists. This means that a certain amount of borrowers must default on their debts and foreclose. The alternative is that other people take out more and more debt to increase the amount in the economy so that you can get your extra bit to pay interest + principle. But this just causes the problem to get bigger, and pushes the foreclosures into the future.
Australia has been increasing it's borrowing at a rate of 16% compounding annually since the 1970s. We're currently at a debt:GDP ratio of 160% and starting to see a record number of foreclosures. Pointing out that 16% growth in debt with 4% wage inflation is not sustainable is hardly necessary.
This probably sounds like I'm making this up. I wish I wasn't. Ask Alex Lee, he works in a bank.