"you can't rely solely on insiders to do the testing"
I can't see anyone proposing that. It needs both an inside testing department for the OS plus the external volunteers.
Getting the OS right surely has to be fundamental, it's the building block of everything else. Using Agile on the OS is completely irresponsible, might as well rename it to Fragile for such a critical purpose.
The Cost Accountants are ruining everything with their short-sighted, one-sided approach. I bet they'd soon change their tune if their Excel sheets were so full of bugs that their formulae and numbers were all wrong or crashed.