One, we don't know what the full slate of NOW! titles will be. Marvel's new "seasons" usually roll out over the course of four to five months.
Two, DC Rebirth is only in its third month. August was still a big month of debuts so it's going to be awhile before we know about drop offs. Especially as stores make their orders three months in advance so August's Diamond numbers reflect orders made before Rebirth even arrived in stores.
Three, the fact that Rebirth is DC's second line-wide reboot in five years says a lot about their long term ability to hold readers. If they could hold on to readers not only would we have not had the New52 to begin with but we wouldn't have Rebirth now.
Good for DC for getting back into the game but this all smacks of deja vu.