I used to maintain two of the four machines that convert the milk powder base to emulsifiable instant milk. There are only two companies that do that in the US. It's all monitored, inspected, and controlled by the USDA. There is a complex commodity system in milk that is a national security and price stabilization thing since WWII. Essentially dairy milk is turned into dry powder and stored in government warehouses. A three year supply exists at all times. Excess inventories are exported. This is not the end product of instant milk or formula. It's the raw material milk powder base.
If Abbott shut down its plant, those government warehouses would be overflowing with milk base in a month with nowhere to put it or the government would be exporting the older inventories to other countries to keep prices stable and safe. The USDA is knock deep in this whole process of national dairy stockpiles. Nothing is a "surprise".