In the seventies Green Line became very unreliable due to traffic delays which wrecked the timetable, but also due to staff shortages at the time. In those days the morning Radio London traffic news would include a long list of Green Line cancellations and regular passengers who could not rely on the service would give up and find an alternative.It mirrors the decline and elimination of Green Line in the 1970s and 1980s - each cut loses regular passengers if they have any possible viable alternative. An unexpected half hour, or even hour's, wait beside a busy 'A' road with HGVs splashing past can be a killer.
I suggest that frequency may be less important than reliabilty.