Because "Use them just twice" isn't a very good option. Neither is "Have the Borg leave scars" because they never did that before, not in TNG. You keep saying that Voyager weakened the Borg, and "Scorpion" is hated by Borg-lovers because it dared to show that there was at least one alien species out there that could beat up the Borg like they were nothing. So having most of the Borg be annihilated by the 8472 would be the same as "weakening them" like how VOY-proper did. Every single time they tried (The Kazon, the Vidiians, the Krenim, the Hirogen) the reaction was always "They shouldn't keep running into these guys!" making it clear that the audience was never going to accept a continuing enemy. Yet they never complained about the idea of the Borg showing up more than once meaning they were the only option for a recurring external enemy.