A 30 minute wait when the new half-hourly rail service actually starts on a cold station is not as good as a 12 minute wait at the Metrolink stop, which incidentally is far more easier for bus passengers to make the required interchange, if coming in from surrounding areas.
What you have to bear in mind is that people turn up at a tram stop whenever they like because of the frequency, but they have to put up with a 30 minute plus journey. With the railway people get to know the schedule, they can check the running online, and with apps, and they can turn up when the train is due (rather than just after one has gone), thus the wait plus journey time for the railway is much less than for Metrolink in most cases.
The vast majority of buses in Eccles go to Manchester anyway, so the interchange theory is a bit of a red herring, especially as many buses are quicker than the tram and more frequent (every ten minutes on quite a few routes).
Everyone in Eccles knows that the tram only goes to Eccles because it's the only way GMPTE (TfGM) could get the necesary figures for a tram line into the Quays.