I believe that communal bathrooms are cheaper to build and maintain, hence we still have them, not because anyone enjoys using them.

You have a point, but originally interest was both compensation for the lander for offering you the loan, and a way to mitigate risk in case you fail to pay. Without interest, why would anyone lend a random stranger some money? Interest is basically the only things that props up an unsecured loan.

Only if you don’t have insider info.