"it was required to keep a second shuttle on standby just so they could use it as a lifeboat"
Only for missions that couldn't reach the ISS. And they could have kept one orbiter operative and the other two as backups, to be used only for specific missions requiring an orbital workshop, or re-entry capabilities for precious experiments.
And I find very funny that in the late 1970s they could handle Shuttle complexity, but in the 2000s they could no longer - nor devise ways to reduce it with new technology. It looks more NASA lost its capabilities, and had to return to simpler devices costing roughly the same. More lack of talent and skills, than money. Maybe the need more immigrant scientists and engineers from Germany again?