And we're forgetting the strength of planetary defenses.
I would think they are the way Gene described them which would be
able to blow almost anything to dust if given the time to arm and fire.
Not as much need to waste resources or manpower of Starships to be
sitting around when you've got that, especialy in peacetimes and taking
into account the location of Earth within Federation borders.
Additionally, we don't know if those two "sub-thingies" as cited above constitute the whole of the defense perimeter; I think it's very plausible that when the line was mentioned, those two ships were all that were left after the Borg trashed the whole line. So I think it's just as fair to assume there was more to the defense perimeter than those ships.
There's even precedent for that sort type of inference in the very same episode: when the Enterprise reaches Wolf 359, we only saw a handful of destroyed ships (six, perhaps?), but somehow that was supposed to represent the full 39 ships destroyed. And we the viewers accepted that visual trick.