To put things in perspective: The problem "It is bothersome to carry all the cash I have" has to be the ultimate first world problem. Seriously! Control question: Can you name one single first world problem that is more ultimate than this?
And secondly, an economic system which allows for a government to spy on every single transaction will be an enormous gift to totalitarian regimes. We as people living in fairly free countries have a responsibility to keep cash as a fully functional alternative and not export such gift to governments violating human rights and persecuting dissidents. Regardless of the lack of "modern" feel to it, the (minor) cost of doing this, or any other reason.