Good points on JIT! W. Edwards Deming would be proud!
JIT's main benefit is to minimize inventory carrying costs throughout the supply chain.
Proper vendor relationship management is critical to keep production lines moving.
There are other benefits, such as avoiding obsolescense, minimizing waste, etc.
As
@Jack Watts points out, some finished goods are better suited to JIT than others.
My point is, a part shortage is a part shortage anywhere in the supply chain.
Manufacturing inventory levels, managed by MRP programs, include safety stock in reorder point calculations.
Companies do have more than one supplier for critical parts, for several reasons including outages and cost competition.
I have been working on MRP, MRP II, Cost Analysis and associated manufacturing process programs for close to 30 years.
By the way, I hate SAP and Oracle ERP packages. Filthy expensive and return garbage.