Is it possible not to be concerned?

At a time when public debt ratios and the quantity of money are historically high, some economists propose taking advantage of the very low long-term interest rates to use debt to finance necessary public investments and public spending (healthcare, training, energy transition, reindustrialisation, infrastructure, etc.). Clearly, they are not concerned about the state of public finances and the abundance of liquidity. What conditions must be met for them not to be concerned? The only configuration in which we may not be concerned is one in which the return on public investment and additional public spending is high enough to remain higher than interest rates on public debt, even if they rise sharply. This is because:  If long-term interest rates remain very low for a long time, changes in the quantity of money (since central banks will have to continue to buy bonds) and in asset prices will be unsustainable; If long-term interest rates rise and if returns on public investment are low, governments will quickly become insolvent as potential growth will remain low. Much will therefore depend on governments’ ability to select high-return public investments and public spending. We can have general ideas on this subject (R&D, education, industries of the future, etc.), but in practice the selection of spending is much more complicated and takes place at a finer level.