Few industries are as ruthlessly grinding as mainstream automobile manufacturing. This makes alternate histories where the “independent” American auto companies stay in business extra-challenging.
Historically, most of the independents were wiped out by the Great Depression. After an artificial postwar spike thanks to demand after a lack of car production in the war, the survivors were forced to consolidate in the early 1950s after a production race between Ford and Chevrolet glutted the market.
Studebaker-Packard was out of the auto business in a decade. Nash and Hudson “merged” into American Motors (in reality Nash essentially kept Hudson’s dealers and obliterated everything else), and were only saved by investing in an inherently counter-cyclical compact just in time for the 1958 recession.
This could only happen once. Domestic compacts and imports moved in to hit AMC’s niche, and they were forced to play an innovation game with few resources for the remainder of their existence.
And that was the successful one. Kaiser Frazer fizzled out simply because it didn’t have enough money.
All this happened before the 1973 gas crisis, and before the bulk of emissions and fuel economy regulations came into effect. Tough business, the auto industry. This, combined with the inability of GM itself, much less a smaller competitor, to sustain a giant multi-brand lineup without large quantities of badge engineering, makes me skeptical of timelines where the independents stay active.