The phrase "unsustainable levels" means that something is happening or being done in a way that cannot continue for a long time without causing problems or running out. Full definition
This number is itself high, which only reinforces the point that dividend - paying stocks have reached unsustainable levels of valuation. (hardassetsalliance.com)
If we've learned anything over the past decade, it's that things can stay at unsustainable levels for much longer than anybody might imagine. (treehugger.com)
These developments have caused concern that families might be reaching unsustainable levels of mortgage debt. (simplifyingthemarket.com)