Your example is exactly backwards! In Massachusetts v. EPA, the Supreme Court ignored Chevron deference, and told the EPA that CO2 could be a pollutant under the statute. The EPA didn’t want to regulate CO2.
Yes, and later on when the EPA made CO2 a pollutant (WEST VIRGINIA v the EPA) the supreme court ruled "Actuaally... JK, ignore that last ruling because you can't do this". [1]
As in, they literally ignored their ruling made just a few years prior because when the EPA called their bluff it would result in an outcome they didn't like.
Your example is again backwards. West Virginia v. EPA said nothing about whether the EPA could regulate CO2. The Court took it for granted that the EPA could do that.
The case was about the interpretation of 42 USC 7411, which creates an express distinction in how new pollution sources and existing pollution sources are treated.
The Clean Air Act created a fundamental compromise: the EPA could require reasonable emissions control technologies on existing plants, and could require more aggressive pollution control on new plants. In this case, the EPA decided that the power to require emissions control technologies on existing power plants gave it the power to require the entire industry to shut down fossil fuel plants in favor of renewables. That was an obviously stupid interpretation of the statutory language that nobody exercising good faith could seriously credit, and which gutted the statutory compromise in the Clean Air Act.
Imagine Congress passes a social media law, which allows the FCC to require social media companies to implement “reasonable privacy protection technology.” The FCC decides that social media inherently risks privacy, and bans your company from collecting any user data at all. You sue, and the FCC says that banning social media companies is a kind of “reasonable privacy protection technology.” That’s basically what happened in West Virginia v. EPA.
Your constructed example would fail even under pre-MQD Chevron because banning companies is not a reasonable interpretation of a "technology". Maybe you find the case obvious (unlike the SCOTUS judges, who were split) because you are missing some its basic contours?
