What happens if everyone saves and there are no debts to invest in? That’s the not so tiny paradox of thrift.
Then G-d fucking forbid, people would need to go to work in a productive industry, invest in a small business, or provide a useful service. Most people will still exchange money for goods or services they can’t provide themselves since they’re not 1880s pioneers.
There would still be short-term debt (0% interest between providing a good or service and being paid) of course.
Housing would still be a decent investment, since not everyone would want to buy a house (especially if it would require actual savings), BTW.
