[scifibestfi]

If everyone treated their credit card like a debit card, and only spent what they have, would the rewards disappear?

[MajimasEyepatch]

Not necessarily. Ever notice how your card usually has a bunch of retailers you can pick to get discounts or extra points from? That's not just your bank being generous. That's usually paid for, in one way or another, by those retailers. They're basically sponsored rewards (I don't know what the industry term is). Credit card companies would still get paid transaction fees and get paid for those "sponsored" rewards, so they'd still be incentivized to make you spend more. But I imagine the rewards would be less generous, and perhaps differently structured.

[supertrope]

No. A reward card might charge the merchant a 2.5% interchange fee, they kick back 1% to the customer. Versus a non-reward card that might charge a 1.5% interchange fee.

[ghaff]

It would presumably still be a competitive market for card issuers.