Obviously the law would probably be more involved than that - but no matter what a law mandating the acceptance of cash would be simple and straightforwards because all the technology and processes for handling cash exist right now and would not need to be defined.

Other laws to protect people's rights in a cashless society seem like they would not be simple, but I am willing to be convinced otherwise, hence (and here I do not think you are really engaging with what I wrote) I suggested that if they had an idea of what these protections would be they should list them - why? Because it is not enough to say oh there should be some system to handle edge cases when there is already a system to handle edge cases but not at all delineate what the features of the proposed replacement system would be!

The cashless proponents, as opposed to mainly credit and small remaining cash usage, are like people who have a large enterprise system written in Fortran that works with very few problems for billions of transactions a year, handles down time and performance problems impeccably, but would like to rewrite the system because one type of transaction that the system handles is becoming less used (even though that type of transaction is the by default secure fallback should other transactions experience processing problems) and this seems like a really great time to modernize our architecture and infrastructure!

This is a prediction, not a historical fact. It is repeated in multiple comments on this HN article. Yet the Federal Reserve reports that U.S. currency in circulation has been steadily rising for decades, including the recent smartphone decade: https://fred.stlouisfed.org/series/CURRCIR

Like others, I incorrectly assumed US was using less cash.

Cash is winning in the world reserve currency, and the cashless doth protest..