The problem, most fundamentally, is that money is one among many variables in the economy, and all are interdependent: each affects and is affected by all others. That is the definition of a chaotic system, one that cannot attain a stable equilibrium.

The solution is to make the supply of money an exogenous variable, one that is not influenced by other economic variables. That is what Friedman was attempting with a fixed increase in the supply of money over time. Any approach that links the supply of money to any aspect of the functioning of the economy cannot be fully exogenous.

I have developed an alternative paradigm for making the supply of money (as currency) a fully exogenous variable. In it the size of the supply of money is determined by demographics.

In this paradigm government--all government, from central to local--could be funded without taxes or public debt. To be clear: both could be eliminated. As with Friedman's paradigm, a steady flow of money into the economy would require that money must somehow be taken out of circulation. My paradigm has a mechanism for directly collecting money to be returned to the central bank--but people and business would be able to retain plentiful pools of money, based on income, and no money would be collected from any person or business before it could be used for purchases/investment. Unemployment and poverty would definitely be eliminated. Environmental sustainability would be increased. The economy would be stably self-regulating.

All of that would be accomplished without imposing any cost on employers, without redistributing anything, without imposing any limit on income/ wealth, without additional regulations, and without requiring people to act any particular way. That, to my mind, is truly a "21st century" approach to money.

If curious, a brief overview is in "Permanent 'Quantitative Easing'" (a "4 min read," with links for further reading, all here in Medium, but not behind the paywall).