It's not instant and the fees aren't always low. I was trying to transfer $10 USD in Bitcoin last May, when the BTC market was going insane, and it was impossible.
I'll even throw in a scenario that I've encountered: the card readers in a store aren't working, because there is a connection issue. Instead of using cash in my literal physical wallet, I'd like to pay with my digital wallet. Can I conduct my transaction offline with the store?
In the scenario you mention, I’d just reach for cash.
However, as a protocol for payments/value in the digital world, it’s very compelling.