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.
I think this is exactly why we need larger blocks, so that there is no need to wait for multiple blocks for a tx to confirm, provided that a CPFP tx includes a sufficiently large tx fee.
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)?
Not at all, however there are similar risks with accepting credit cards, however these risks can be mitigated with CPFP, and if the customer is still in the store then the manager can speak to the customer about the fact that their transaction is not going to confirm (which is not going to be the case for a customer whose "transaction" "fails" up to 180 days later when a credit card dispute is filed)