You don't understand payments if you think they can just replace Visa/Mastercard. The whole value add of Visa/Mastercard is the networks they have with the hundreds of thousands of banks, etc, which Stripe doesn't have.
Stripe only partners with a merchant acquirer bank. To replace Visa/MC they'd also have to get into the card issuing side of things: get a bank or banks to issue "Stripe cards" and get merchants to start accepting them.