Essentially I think this gets into a estimation of magnitude problem (outside of ethical, security, and other concerns). Where if the payment isn't ever the action being waited on, a contact-less payment while convenient doesn't save you any time to get out of the store. If you have just a snack and no lines then the payment will be the action causing the bottleneck.
Pretty interesting parallels to biology too, like in some birds there's the race towards sexually selecting ever fancier tail feathers that might cause more female birds to ooh and aah perhaps, but hurt your chances of flying away from a hawk, and lead to an overall reduction in fitness in the species as the hawks start finding more success and expanding their population size at the expense of yours.