In these days, it is usually described as a loss of “mojo”, although I’ve never been certain of what that word actually means.
In terms of the launch of the 307, I’d prefer to describe it as a fall from grace. I suppose I could also have picked the transition from 205 to 206 from the same stable, but I think it less obvious and memorable for me. I think I need to become instantly more specific. The 306 was the chassis benchmark in its class. It was also one of the more lovely looking mid-range hatches of its time, but I think aesthetics are much harder to benchmark, and I am certainly less comfortable opining on the way a car looks under such a heading.