There is nothing 'modern' at all in Modern Monetary Theory. It is the same old fish wrapped in slightly different paper. The godfathers of Modern Monetary Theory are John Law, G.F. Knapp, J.M. Keynes, and most lately Alan Greenspan. But its roots go back to any ruling group that ever debased a currency or seized private property by fraud.
It is in the nature of a Ponzi scheme. As long as its sphere of influence can keep expanding, and the force by which people are compelled to accept it is maintained, a fiat currency will 'work.' But as its expansion slows, as outlying regions begin to resist it, the currency begins a slow but deadly spiral of collapse that accelerates into a final reckoning and reissuance.
Fiat currencies *can* work well in theory, but in reality they require the indefatigable dedication of people of extraordinary virtue, courage, and wisdom. And so they have failed. Always.
And it most certainly will not work with the craven and self-serving leadership which the Anglo-Americans have today. It is almost a cruel joke to promote such a system. And yet there it is and here we are. What comes next will be interesting.