It's not about a couple of spares vs a warehouse. Spare inventory levels are determined by expected failure rate x install base and also the expected lead time

It's not about a couple of spares vs a warehouse. Spare inventory levels are determined by expected failure rate x install base and also the expected lead time to buy more. But when the entire product gets recalled then suddenly you need to replace every instance at once, and that's not a thing you ever buy spares for. Even if you had multiple suppliers every manufacturer is also placing orders all at once for the remaining suppliers and they don't have the production elasticity to respond that quickly. Supply chains are complex systems and it's never as simple as "buying more spares" and "having multiple suplliers".