Post-scarcity doesn't mean post-danger. No one has to do luge in the Winter Olympics to survive, and not more than 4 years ago someone died doing it, and yet people continue to do so.
It naturally follows that Earth was never in serious danger at any point because, hah, are you insane? We just didn't hit the Borg cube with the Lance of Judgement and instakill it from half a galaxy away because that would have been a terrible affront to Picard's character arc, and we really care about that sort of thing. Look at our Prime Directive: we'd rather genocide an entire race than interfere too much in the story. (Heh, psych, we're not total psychos. The Prime Directive strikes a few chords with our culture, because it needs to sound plausible to Starfleet officers, but isn't our main guiding principle any more than the US is based on not violating a shot clock. The Prime Directive is actually just there to provide interesting moral situations for Star Trek officers. We wargamed out several hundred generations of officers on a Holodeck simulation and they end up tremendously unfulfilled when doing the right thing is obvious, possible, and successful, like it always is in real life in the Federation.)
That is a beautiful, beautiful thought and plot device for a Sci-Fi book. Thank you for placing that into my mind.
I love it.