Look, I've never been a fan of the Detroit designs, they've never been designed with anyone under 6' in mind. But it wasn't the design that sank them - it was the gas mileage. Auto manufacturers all over the US had the same problem and yet they survived. Some of them with little to no immediate design changes.
As I said initially, it wasn't just the unions, though they were a huge part of it. It was also the corrupt local political environment that wortked with the unions ensuring they paid each other off.