You're not supposed to use the $10B to buy treasury bonds.. you're supposed to use it on R&D, manufacturing, QC, etc...
The parents said cash on hand.
You are proposing a third hypothetical. Would Boeing being a better position today if they had spent 10 billion dollars yesterday? Maybe, maybe not.
Who decided that's what they were supposed to do with it?
The fact that they are having to sell these shares to try to right the ship means the market decided. Otherwise they could just keep extracting as much profit as possible.
Oddly enough people that fly on planes want to be confident that the planes will work.