That is a sharp question. You are right about the opportunity cost. As a banker, I look at the "Depreciation Period" (Lifespan) of the project.

If you are building a "Pop-up Store" (a prototype or script), use libraries. Don't waste time on craft. But if you are building a "Shrine" (Core System/Database) that must last for 20 years, "Shugyo" is actually the cheapest option.

Efficiency is cheap now, but expensive later (Technical Debt). Craftsmanship is expensive now, but cheap later (Stability).

We don't need a Jiro to run a fast-food franchise. But we need him to build the Kernel.