Of course it's not deterministic. If 100% (or even 15%) went bankrupt, no one would finance them. It's impossible to get to this point.
Every deal is a snowflake, but the reductive caricature of Bain and "private equity leveraged buyouts" is: "(1)buy a company. (2) get your money back quickly by borrowing on behalf of the company (3) profit on your free option.
This caricature is not that far from the truth, as caricatures go, and it's a pretty good guideline to speculations about what they plan to do with Virgin Blue. My guess is that a covid-related bailout or stimulus will provide the capital.
Again, situations differ, but generally these deals don't capitalize a company... lenders do. What they bring to the table is risk tolerance and a mercenary attitude... much of it related to the free option they are exercising.