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).
Ciyam.. as you are a person who was involved in the blockchain.info project, im certain that even you know that offchain tx's is a very easy thing to impliment, to send tx's instantly between users without touching the actual blocks..
after all people dont do bank transfers for a cup of coffee using swift. they use third party services such as visa. so offchain is a solution.
why do you think apple now want you to pay with applepay for itunes.. because its alot easier then asking for 70c from visa everytime..
secondly its easy to check a receiving TX, see what address it came from and check if the originating funds already have confirms to not delay things in regards to the new tx the shop just received.. plus if your going to do it. you would also push the TX to all the miners, just incase the customer has only pushed it to a explorer service nodes in attempt to try look like its been received but still not quite entered the mempool of miners.
im kind of surprised your coming up with these objections, especially with your experience