Show me an episode of TOS, TNG or DS9 where Kirk, Picard or Sisko run into a world under attack from some enemy and they just decide to leave them to their fates either because they were scared or because they didn't think they could do anything.
And yes, the crux of the matter with newtype_alpha WAS that they could fight the Borg.
I spoke about the 8472 aliens with guys on forums like rpg.net, at least there they were more open to the idea that Borg were only good for 2-3 stories and TNG already used them up beforehand so they didn't blame VOY for the decay.