[DrChief]

I've always interpreted the whole Kamen Rider powers thing as one where the events are erased since the most necessary rider no longer exists. Brave may have been one of the first Kamen Riders to combat the Bugsters, but it wasn't until Emu became Ex-Aid that we would eventually learned of the truth behind the Bugsters, and actually managed to defeat the main antagonists who would of perpetuate it.

If Eiji hadn't become OOOs, then there wouldn't be anyone suitable enough to fight against the Greed in part due to Maki's interference. If Ryotaro hadn't become Den-O, then the Imagin would have been able to continue wrecking time at their own pace. If Gentarou never went to Amanogawa High School and became Kamen Rider Fourze, then there would never be that high-spirited kid who would manage to gain power via friendship and defeat the Zodiarts.

The main Riders are the ones who end up setting up the events that would lead to the defeat of their main antagonist groups, in part due to their outlooks and capabilities. Without them around, chances are victory wouldn't be as high against these villains.