What is the cost to restart both the supply chain and production line? If this lasts long enough, then there are portions of the supply chain that will start to simply disappear as vendors go under or switch away to different customers and de-prioritize Boeing. It isn't just the monetary cost to re-calibrate machining and tooling, also the staff time and monetary cost to fill in gaps that appeared during the shutdown…
Where would they put them? These things ain't small.
Airliners are expensive. The manufacturers do not build a large number speculatively and then find customers. They find customers, then build what has been ordered. They have enough storage space to serve as a buffer between output and delivery so that minor glitches on either don't affect the other.
Also because airliners are expensive, and they get paid on delivery not on order, once past the initial production current production is paid for from recent deliveries. If deliveries have to stop for an extended time, there is no money to fund more production.