They can threaten to do that, but it wouldn't work. Northwest has about what, 5400 pilots? What do they fly? B-747's (200's and 400's), B-757's DC-9's, A319/320's, A330's, and DC-10's? A lot of training on a lot of different airframes, including three positions on two different fleets.
Assuming the pilots walk, Northwest (or any airline) for that matter cannot possibly hire and train that many pilots fast enough to survive.
First, the airplanes do not fly, the revenue does not accumulate. If they were to hire scabs off the street, it would take them 3-5 days just to go through indoc. 3-5 days of airplanes just sitting around would sink them, especially given their currrent financial situation. You can hire current/qualified people, but they can't be put into the cockpit overnight. By the time the scabs are trained and ready, there wouldn't be anyplace to go.
Second, even though some managers would fly, it isn't nearly enough to cover the system, probably not even enough to keep the place from imploding on itself. Don't count on the regionals to pick up the slack either, they already have their hands full with their own flying. Besides, the most profitable international routes cannot be flown by the regionals anyway.
Third, PR. A strike isn't good PR under any circumstance. People hear of a strike and of an airline scrambling to replace it's pilots, the first things to come to mind are going to be safety and reliability. There is no shortage of competition out there, and most travelers would simply flock to other carriers. And don't think for a minute that the competition wouldn't add additional flights and airframes to cities impacted by a labor dispute.
Fourth, training costs. Where is NWA going to get the money to train 5400 scabs? They would bankrupt themselves just paying for the training expenses.
