But what if the whole thing goes belly under?

The speaker, who is a flight attendant, was talking to her son about possibly taking a voluntary leave of absence in light of covid-19, but fearful about the financial status of her airline company that she works for.  This is a nice congruent conflation of “going under” and “going belly up”, both referring to a business that goes bankrupt or cleases to exist.  This is a directional (“under” vs. “up”) mixup, common in the malaphor world.  A big thanks to Jody Compton for uttering this one, recognizing it as a bona fide malaphor, and sending it in.