Paying the merchant before the customer's card payment settles would mean advancing funds, which would start to resemble lending/guarantee rather than payments, raising regulatory issues. It would also concentrate risk at the platform: defaults from one merchant’s customers could jeopardize the platform for all merchants.
Of course, this would then mean that the customer is trusting merchant not to run off with their money.
- The customer has to pay upfront, which lowers conversion rates.
- No shared balance across multiple merchants, resulting in higher total payment processing fees.
- As you already noted, trust shifts to each merchant to honor unused balances.