Under what conditions is a long period of simultaneously expansionary fiscal and monetary policies acceptable?

An increasing number of OECD countries (for example the United States, France, Italy and Japan) are conducting expansionary fiscal and monetary policies. This brings to mind the recommendations of modern monetary theory (MMT): maintain full employment with an expansionary fiscal policy and prevent interest rates from rising by using money creation to finance fiscal deficits. This kind of policy was rejected out the past: Continuous fiscal deficits led to a loss of fiscal solvency; Continuous monetary expansion led to inflation and to asset price bubbles. But today: The new functioning of labour markets results in persistently low inflation; The absence of inflation allows monetary policy to remain expansionary, and therefore long-term interest rates to remain low, which eliminates the risk of a loss of fiscal solvency; The preference for risk-free assets stops bubbles from forming in risky assets (equities, real estate); A “bond bubble” (abnormally low long-term interest rates) is therefore the only cost of this policy. But abnormally low long-term interest rates are dangerous only if these interest rates rise in the future, which will not be the case if wage inflation remains low and the preference for risk-free assets remains strong.