Investors have been disappointed too often to give car manufacturers much advance credit for cost-cutting plans. In the case of the industry’s big merger, that may create an opportunity for those that dare to break ranks.

The 2019 financial results of PSA Group, the French company due to merge with Fiat Chrysler Automobiles, again showcased the profitability for which it has become known. Its operating margin last year was 8.5%, up from 7.7% in 2018 and well ahead of peers in both Detroit and Germany. Its shares rose 7% in…