I apply to the bank for the 100K mortgage. Forget the interest. The bank buys the house from the seller with old money. That swaps one asset (cash) for another (house.) No change. The bank writes a mortgage contract, has me sign it and turns the house over to me. That swaps one asset (house) for another (note receivable.) No change. In my case I use old money to pay the mortgage until the contract is satisfied. I have the house now. I bought a house that came from old money from the bank with my old money. The seller swapped his asset (house) for another (cash.) I completely lost the new money.
Banks don't collect and lend out old money. That's just not how they operate. When you apply for a mortgage, the bank marks up your account by $100K, and you execute a $100K+ promissory note; bank assets and bank liabilities have both gone up by $100K, and so have yours. Then, the bank also has to adjust both its reserve balance and its capital account balance up by $10K.
You pay your mortgage payment with old, pre-existing money - money that was borne of somebody else's loan at some earlier point. Maybe your employer takes out loans in the normal course of business. Anyway, as you pay off your loan, your loan amount decreases, so bank liabilities decrease, and the number of bank-created dollars decreases. When your loan is paid off, M1 is back to what it was before. Of course, other loans are being made and paid down every day, so we always have a supply of bank-created dollars that stays fairly steady.
Back to the interest. The bank took a risk by writing the mortgage and earned a profit (interest) by doing so. That is the change. It is the only thing I can see economically changing for the entire transaction. The real estate transaction itself had nothing to do with it that I can find.
The interest is just extra money that you pay the bank for the benefit of getting the loan. The net of your loan is that once your loan has been extinguished, M1 has not changed, and some old money has moved from you to the bank. That is their profit.
On the other hand, if I dig up some valuable rocks, I have gained an asset without spending anything other than my effort. I have brought some new wealth (if not money) into the economy. I have something new that wasn't there before. The economic change of bringing a new asset into the economy seems intuitive to me. Not to you?
You digging up some valuable rocks is no different than if you worked and earned money from your employer. Your work is still paid for with old (to you) money.
The economy benefits from the simple act of you working and being paid for it. That's commerce, and wealth is created - a house gets built, you dig up some diamonds, farmers grow food, etc. But the whole point of the conversation is that if you earn $50K, the most you can spend (without credit) is $50K, and this is true of the nation as a whole. Without credit, you are maxed out at $50K of spending. With credit, you can spend, say, $55,000, and the extra $5000 becomes income for other people and businesses. If lots of people and businesses also use credit, you have the opportunity to earn
more than your normal $50,000. That is how economies grow.