Before: Alice wants to buy a chair that costs $100 from Bob. Alice saves $10 a month and buys a chair. Bob got $100. Alice can buy a new chair every 10 months. She may also conclude after 10 months, that she doesn't want the chair that badly, and would instead save the $100 for retirement, or pay it into the house mortgage, reducing the amortization by a week.
Now: Alice wants to buy a chair from Bob. She gets a loan, paying $10 over 12 months. Bob gets $100, Affirm gets $20. Alice can now buy a chair every 12 months. Affirm's founder buys a supercar.
Slightly later: Alice's job gets cut due to COVID. She can now only pay $5/month. She gets a $100 stimulus check, but instead of paying off the chair, she buys Affirm stock and remortgages the chair to 24 months. Now Affirm gets $40 in interest and $100 in fed money routed through Alice.
Net effect: Alice's purchasing power has reduced 2.4 times, Affirm's founder buys 50 new yachts and starts a company that lets people get loans to buy food.