What you say is most likely true, ie.the bank wouldn't accept your offer to clean their windows.  However, what you can certainly do is go to another bank and tell/prove to them that you have a started a new window cleaning business that is profitable and because of this predictable income get them to pay of the existing loan by taking out a new loan.  This, by-the-way, is called refinancing a loan and is done all the time.

In case you didn't know: supplying, buying and selling debt is the biggest business on the planet!  Without debt almost none of our economy would exist.  Over history the use of debt, (ie: the ability to get something now and pay for it later with interest) is the way by which we have financed the development clean drinking water, abundant food, education, modern medicine, modern transportation, computers and telecommunication, etc, in fact just about everything that you use.   Note that the countries with the least developed banking services are the most backward, undeveloped third world countries that exist.  There is nothing wrong with debt that is used wisely.

Regardless of all this, the whole point of my original post still stands: I supplied an example which demonstrates cleanly and simply that the claim that it is impossible to ultimately pay off interest in a world with a finite money supply is NOT true.