I spent my first summer working in a bicycle shop the year Elvis died - Purdue's International Sports on Westheimer, near the Galleria, in Houston. George Purdue was my boss.
The 10 speed boom had just sort of ended. For the industry, sales were really cooling off, but people were riding bicycles like crazy. Cross country bicycle touring was at its peak. Bike Centennial had published cross country tour maps encouraging folks to cycle through the countryside, coast to coast, to celebrate America’s 150th birthday.
But bicycles from that era had a series of chronic problems. Among them, gear troubles... The derailleurs (gear changers) were made of a composite of pot metal and low tech plastic. In fact, gear problems from 35 years ago still haunt this industry today. Old timers, that were skeptical then, only had their fears reinforced by the slipshod gears of yester-year.
Everything has changed since then. Gear shift systems today are awesome, reliable- there is no comparison between then and now!
In a more subtle area, there were developmental stages in the bottom bracket and crank area of the bicycle. Some of these changes effected the frame area of the bicycle.