most things like cups of coffee in starbucks wont need confirms or reversibility.. because the coffee buyer can slap the barrista in the face with a wet fish if the customer doesnt get their coffee after payment.
Actually in the specific case of something like a cup of coffee it is the vendor who will be taking a big risk if blocks are near to full (be they 1MB or 10MB) as there are multiple threads about txs that never confirm.
How hard would it be to write a simple little wallet that purposely crafts txs that won't confirm (before they are dropped from the mempool)?
As soon as that kind of thing is reported (assuming it does occur) then you will see merchants spooked and stop offering Bitcoin as a payment option (which is also why I don't see any urgency to push for Bitcoin to be used as a general purpose payment system).