There was a lot more competition, even when GM had 50% market share. Back then, even GM divisions were competing against each other. From the 20s to the 60s, there was extreme competition just in the domestic market
You had GM with 6 or more divisions depending on which year you counted.
You had Chrysler with 5 divisions.
Ford with 5 divisions.
Packard - Studebaker
Nash - Hudson - Metropolitan / AMC
Kaiser Frazier - Henry J - Willys
International
etc etc etc.... and that's before any of the imports.
Today we have a GM that is down to 4 divisions with relatively little overlap, Ford with just 2, Fiat Chrysler which is a mess except for Jeep and Ram.
Today the Ford Fusions has domestic competition from the Malibu, Impala, and Charger (if we're being generous). That's before all of the import competition in that segment.
I'm not sure I could count the number of competing domestic products for the Ford Fairlane range. Ford had at least 4 competing products from Mercury and Edsel, GM probably had another 6 if we count everything from Chevy to Olds. Another 6 from Chrysler. That is 16 competing nameplates from just 3 companies.
But once imports started gaining a foothold here, the domestic companies could no longer fund that inefficient style of operation.... did GM really need however many different V8 designs it had in 1958? I get that some variation is warranted, but a completely different block between a Chevy, Pontiac, Olds, Buick, and Cadillac is just horribly inefficient. Cut that down to 3 blocks that all divisions can use and change the tuning or cams to allow for brand specific needs.... tada! That's where we are today (5.3 liter, 6.2 liter, 4.2 liter)
In order to reach that economy of scale, the manufacturers would have to reduce their portfolios to just two or three platforms and try to cram everything on just those platforms.