I've been trying to be cashless for a few years now, at one point i even managed to put my cards inside my phone, under my battery cover. when having too little cards was impossible i bought a silicone cover for my phone and put the cards there. You can have an "sos" bill under your phone's battery cover too.
The reason i'm so hyped about the cashless society is because of the social implications. If cash is cancelled by law, crime will reduce significantly, stolen stuff will be alot harder to sell, including illegal stuff like drugs.
The technical solution imo is an rfid chip inside the phone, it does not need battery and visa is already working on making an rfid card to be put inside a micro-sd card.