I understand that inventory is evil but how does supply chain really solve it? I still have to keep X number of unit in inventory (aka "buffer") to deal with fl

I understand that inventory is evil but how does supply chain really solve it? I still have to keep X number of unit in inventory (aka "buffer") to deal with fluctuating demand. A top answer says that Apple now keeps only 2 days worth of inventory which is quite impressive. If suppliers somehow can do "on-demand" same-day manufacturing then so can I (no?). I would in fact think coordinating between various parts of supply chain will cause inherent delays which will in turn increase the inventory you will need to keep. The large complex supply chain is kind of anti-thesis of monolithic gigafactory concept by Musk. One may argue that Musk isn't successful but I would say with enough capital he should be able to scale up and might get same results as Apple.