A service could pop up that offers to double spend your $5 transaction and refund you a percentage of the amount. If they mine the block you scam a few bucks, if they don’t you paid full price. It would be profitable for the miner and for the burrito buyer at the expense of the burrito seller. If such a service becomes common, that seller is going to start making you pre-order your burrito 20 minutes in advance – at which point people will just switch to cash, credit, or a blockchain without these problems.