Chargebacks are a feature for settling disputes, not a bug. Payment is still instant unless there is a dispute.
If that was the case a merchant could withdraw cash immediately after someone paid them with a credit card. Which patently isn't the case. So no, Payment cards are not instant.
Let's say that merchant accepts payment via Visa on Day 1. On Day 2, Visa goes out of business. The merchant will never receive the payment. So no the payment is not instantaneous. The merchant receives an IOU from Visa that settles in a few days. IOUs != settled payments.