How will the merchant solve this thing if used as currency which needs fast transaction?
Answer 1.
If merchant knows who you are, they don't need to wait for confirmation. They can prosecute you for theft if you fail to pay.
Answer 2.
If the price of the product is low, the merchant does not need to wait for confirmation. They can take a small risk of you committing fraud with the knowledge that 99.99% of customers will not. Same as taking small risk of counterfeit fiat or of credit card fraud.
Answer 3.
If the customer does not use RBF, the merchant can reduce risk by monitoring the Bitcoin network to make sure that the proper fee is paid and that transaction has propagated well. There is VERY SMALL risk that the transaction will not confirm eventually.
Answer 4.
If the customer leaves a deposit with the merchant ahead of time, then the merchant can subtract the cost of the product from the deposit and/or can return the difference to the customer.
Answer 5.
If the customer leaves a credit card registered with the merchant, then the merchant can use the credit card as a backup in case the bitcoin transaction is fraud.
Answer 6.
Use lightning Network.
Answer 7.
Use centralized services such as Coinbase or BitPay to manage payments. Service takes on risk management.
Answer 8.
Smart merchants, and smart service providers will come up with more and better options. Ingenuity, invention, creativity, and discovery are how progress happens.