there are some scenarios where the transaction is failed, especially when the network is congested.
The transaction does not fail just because the mempool is congested. Provided it has been signed correctly and broadcast above the minimum relay fee, the worst that can happen is that it is delayed. Even if it disappears from the mempool, the sender, recipient, or anyone else for that matter, can choose to rebroadcast it at any time. The only time it would fail is if a competing double spend transaction was mined first.
They're too used with convenience offered by digital fiat wallet (e.g. instant confirmation)
We know once we swipe that credit card, that's it.
I think this is a mischaracterization of what is happening. I agree the people
think that it is an instant confirmation, but it isn't. The credit card transaction shows up immediately, just like a bitcoin transaction is broadcast immediately, but it takes weeks or even months for the credit card transaction to "confirm" in the same way a bitcoin transaction would after ~10 minutes. It takes several days for the money to actually be transferred to the recipient, and it is significantly easier to cancel a credit card transaction than it is a bitcoin transaction. I can phone up my credit card company up to 120 days (or sometimes even more) after a transaction and say my card was lost, stolen, cloned, duplicated, hacked, I was overcharged, didn't buy the product, etc., and cancel a transaction.