China’s move to ease trade tensions with the U.S. by scrapping joint-venture rules for auto makers could open a path to higher profits for American car companies, but some said untangling a web of partnerships forged with Chinese companies over decades may prove too difficult.

Beijing responded to U.S. calls for a level playing field in the world’s biggest auto market with a plan to phase out rules requiring foreign auto makers to share factory ownership and profits with Chinese companies by 2022. Regulations concerning...