Boeing's biggest commercial customer, as I understand it, is Southwest, who owns about 5% of all Boeings in the air [0][1]. Thus, sure, Boeing doesn't need any one particular airline, but they certainly do need somebody to buy their planes, and each individual airline which cannot stay aloft is a potential lost customer.
Because planes are so expensive to manufacture, Boeing and Airbus typically take orders for aircraft in advance, presumably along with payment, in exchange for a hefty discount of over half off the advertised price [2]. Additionally, they take too many orders, expecting that they are overbooked on plane orders and some will be canceled [2]. This sort of accounting works well in times of trouble, as long as the system returns quickly to normal following the initial shock. For example, after 9/11, air traffic resumed after a week [3] and there was only about a 30% reduction in air travel, which lasted for about a year [4]. However, if there is a months-long depression in air traffic, then the reduction will be more severe; it's been about 45% to 90% reduced around the Pacific Rim [1] and could get worse.
Eventually, too many orders get cancelled, and production has to scale back, and once production starts diminishing, it's hard to regain capacity. The original article quotes the CEO as saying that it could be 3 years until we get back to prior levels of air travel; that is a long time for airlines to hold out, and a long time for Boeing and Airbus to keep getting orders.
[0] https://en.wikipedia.org/wiki/Southwest_Airlines_fleet#Curre...
[1] Read my cousin reply.
[2] https://en.wikipedia.org/wiki/Competition_between_Airbus_and...
[3] https://en.wikipedia.org/wiki/Closings_and_cancellations_fol...
[4] https://web.mit.edu/airlines/conferences/DC-2002_documents/0...