You're correct, but that still doesn't address the quality control issue for things like metal bending, unless you add intense $$$ inspection to the process. You still have X amount of inventory of items you haven't yet assembled into cars or whatever for the final test of quality.

OK, for some of the upstream problems you could spend extra money to statistically pick individual items out of a designated stockpile/buffer and assemble them.

But stockpiles work better for stuff that can be tested, like piles of ores and elements or at the complete opposite end chips and electronics that can be tested , and nowadays we have the transistor budgets for self-testing.