Boredom

Its possible the economy could fall apart. As sad as it seems, interest appears to be one of the pillars to society.

Without interest, people wouldn't study finance...

I think there would be no banks, credit card companies, or financing, because they'd all go out of business because they couldn't offset the cost of defaulters with the interest of non-defaulters. Even with no defaulters (or, with banks, even considering that you have to have collateral), they wouldn't make any money so they'd have no reason to exist.

Money is make believe and interest is quite literally pulling money out of thin air, if it didn't exist banks/lenders would either collapse or (more likely) develop a new scheme for fleecing everyone

Capitalism would break down. Meaning this sort of economy would fatally crash.