The problem is resilience. Our economy is now fragile, which is not great. And having no excess inventory doesn't mean having no inventory, since supply chain disruptions aren't necessarily immediate to recover from.
Toyota, which pioneered lean manufacturing, is currently laughing its way to the bank; it has a stockpile of semiconductors, since the 2011 Tohoku quake and tsunami exposed how fragile its supply chain for semiconductors was.
Sure, but that still is less efficient when things are working fine. Any time you tie up resources that you don't absolutely have to is a loss of efficiency.
And at the same time every order you're not filling because of an overzealously tight supply chain is a loss of business. Penny wise and pound foolish.
If it makes sense to stockpile for the company that pretty much invented lean manufacturing as we know it, it's not without merit.
Cars are very high cost items. They probably have substantial margins on them. On the other hand, a product that has very low margins is going to find it much more difficult to stockpile.
In this case isn't it needed to consider efficiency over the short and longer term? And for the long-term efficiency one of the parameters would be resilience to shocks, which affects the short-term efficiency (more inventory juggling) but is healthier over a longer span of time.
Short-term efficiency is there all the time. Shocks are not. If the space you're competing in has enough competition then the margins will be low. The companies that focus on short-term efficiency could end up outcompeting the companies that keep large buffers.
Toyota, which pioneered lean manufacturing, is currently laughing its way to the bank; it has a stockpile of semiconductors, since the 2011 Tohoku quake and tsunami exposed how fragile its supply chain for semiconductors was.