Why would it go down? The person is becoming more productive? Do employees at Apple salaries go down because the iPhone they're working on is worth less every year?

Again, tie it to things that decrease in price over time.

Let us say a farmer had taken out a mortgage in 1928, and let us say his mortgage payment was US$20 (equivalent of 1 oz. of gold). In May 1929 he would have had to have sold 114 pounds of cotton to earn $20 (or 18 bushels of wheat, 23 of corn, 44 of oats). By May 1932 he would have had to sold 369 pounds of cotton (or 38 bushels of wheat, …):

* https://www.sciencedirect.com/science/article/abs/pii/030439...

* https://econbrowser.com/archives/2012/02/why_not_abolish

If he had 4 farm hands and paid each $5 (total $20), that's a lot more crops that need to be sold to cover payroll; or he could cut staff.

And it would have been the same for selling any good or service: to pay whatever debts you had (mortgage, car/business/student loans) you would have to work more to earn the same amount of money. Or a company that makes widgets needing to pay the same wages: sell more widgets to cover payroll, or reduce payroll (per head, or total heads).