These emissions regulations don't cover just California, but most of New England, the Pacific, and the mid-Atlantic states as well. I think New York has a way to "fast track" new emissions laws from California, so that the states are kept in sync.
"California cars" were a thing in the early days when emissions were difficult to comply with. Even modern cars come with a "50-state compliant emissions" line item on the window sticker.
If California ratcheted up emissions standards too high, there's the possibility that such segregation could emerge again, particularly when it comes to trucks. There are already several trim levels of trucks that are specific to Texas, because the market there is so big. I could see a truck coming with a sticker that says, "RAM TRX can not be registered or sold in CA, WI, NY, [...] due to emissions restrictions."
