The reason to never charge interest is actually a very physical problem with money.

The inverse is easy to show. Put an ounce of gold in a bank at 0 AD, at a mere 4% interest, and today, you would have more gold than all the mined gold today. This physically can't happen. (thus why we have fiat, so you don't notice the edge conditions)

In a fractional reserve, debt based fiat world, if everyone paid off their home loans, there wouldn't be enough money to do this. All of the money in the world can't pay off all the debt. Today, it can't even pay off part of the debt.

The same thing happens when you charge interest on a physical money (hard money) If you start charging interest, you quickly get into a place where more interest is needed to be paid then all the money in the world.

We will have the same problem with bitcoin.
And we will need to learn that getting a bitcoin back, after lending a bitcoin is a win. Now the world has the bitcoin AND whatever it was used for (like building a house or bridge) Such is deflationary currency.