It could be true that there was some domestic monopolistic behavior going on, in that the steel industry's shareholders were blocking investment into research and modern process development, and since they faced no direct overseas competition due to protectionism, they got away with it for a while. Hence your argument about protectionism being a bad policy has some merit, but only if there's no robust domestic competition driving innovation and better technology (see SpaceX vs. ULA as an example of how competition drives progress).
Similar accounting-based cost-of-labor reasoning was involved in not investing in high-grade silicon and computer chip manufacturing facilities within the United States, although the consequences (supply chain vulnerabilities) seem to have caused something of a panic as seen with the 2021 & 2022 CHIPS Acts, which are probably good ideas, though no doubt they could also be called 'protectionism':