I do not think that is completely true. In the 50's and 60's GM would redesign a car about every 3-5 years... some cases in point, the 55, 56, 57 Chevy, the 63, 64, 65 Riviera, 67, 68, 69 Camaro/Firebird, 66-70 Riviera, They started stretching things out to save on costs, in the 70's & 80's 70-81 F-body, 82-92 F-body, 79-85 Riviera etc... The 80's and 90's were the same... They would have continued the 4th gen F-body for a few more years if the car was crash worthy, they dropped the 4th gen not because of any other reason that they dropped the last generation Eldorado... it was no longer "safe" by 2003 standards. In many cases they simply redesign sheet metal, to make it look different, but with no actual change to the car, like the 97-07 Chevy minivan, from the Venture to whatever they called the last 3 years.
You look back in the 1950's and body styling changed drastically every year... Anyone can tell the differences between a 55, 56 and 57 Chevy from year to year, without much help.
The more frequent changes have to do more with the improvement with rapid prototyping, and the changes the Govt has mandated on crash, fuel economy, etc....
Did you know that there is a regulation that the seats in any car have to weigh so much? It has nothing to do with safety, but is strictly based on the weight... Stupid regulations like this hurt the industry, and slow it down...