Today I bought some chips with 2 quarters, quickly getting out of the store to catch the bus. I love Bitcoin, but it's not going to replace being able to quickly hand in two quarters and then back out. Small purchases like those. I mean if I payed with Bitcoin, or even a debit card, which isn't accepted at the grocery store would have been much more inconvenient.
Why exactly not? With SegWit, Lightning Network and possible even more awesome upgrades to bitcoin protocol, transaction will be almost instantaneous.
And in that situation, you described if you would have bitcoin wallet in your smartphone installed you could scan QR code any cashier can generate and pay in matter of seconds.
We already have technology for that - all we need is will of people to know about this and change current system - which can be much harder.