Before we all get carried along by the beguiling myth that Chrysler really didn't need any help, it's worth considering the bits of the product delivery you don't see: the factories. Yes, many of the designs date from pre-FCA, but a product design is only half the job of making a car. The very first thing FIAT did with Chrysler was pour money into the former Chrysler facilities. From simple stuff like clean floors, proper lighting and increased maintenance spending to reinstating a proper end-to-end quality program (Chrysler had had such a system pre-merger, but Daimler neutered it and Cerberus never bothered).
All of this investment has allowed FCA US to build its existing products profitably, by improving the efficiency of the factories and reducing wasted spending (whether that be rejected components, line downtime, injury events, supply holdups, it's all the same. The WCM system used at FCA counts all of these as "waste" that has to be reduced).
PSA, meanwhile, followed a path that was basically Sergio's original plan for FIAT: aggressive platform and power train reduction, huge increase in commonality but differentiate on styling and branding, and also become a supplier of power trains, not just a captive customer (FIAT, like Peugeot sold engines to other manufacturers). It worked for PSA, it would have worked for FIAT too, but the financial costs of standing the US operations back on its feet prevented the heavier investment that would have been needed in Europe to achieve the same success.
