From NYCT's Staff Summary on the service change:
"The resulting service plan provided a net increase in Astoria Line service, albeit with a decrease in frequency at local stations during the peak hour. Since approximately 43% of Astoria Line riders board at express stations, it was believed that the express/local operation would provide sufficient capacity on the N and W trains.
"In practice, since the service commenced in July 2001, the expected operational benefits of the express/local operation have not been realized, and the Astoria Line component of the service plan has proved problematic from a customer perspective. According to traffic checks conducted in August 2001, the ridership split between the N and W routes was not as even as expected, with N trains carrying 1.9 times as many passengers as W trains in the mornings, and 2.6 times as many in the evenings. As a result, a number of complaints were received from local riders, who experienced long waits and crowded conditions on the N, with lightly loaded W trains passing by on the express track.
"Immediately following the September 11 World Trade Center attack, N service was suspended, and the W operated via the local tracks at all times. When N service was restored in October, the W continued to operate local for several weeks on a trial basis. Checks conducted during this trial period found that the ridership split between the two services was more even, and that the termina was able to accommodate the all-local operation."
In short, in practice, more people were being inconvenienced by express service than were being helped. It's entirely possible that if express service were to be put in place on lines that do not have it (say, Woodlawn, for example), the same situation would occur.
David