Then they should've had some discussion about it at some point, something like "These Borg creatures were once considered out greatest threat. Nowadays we know that they're just one of many."
They weren't, so why would they say that they were? Of all the threats Q alluded to, the Borg WERE the only actual threat to them
because the Borg were the only ones who knew about their existence and had any reason to come after them.
Why does everyone forget the Q as beings more powerful than the Borg and possible threats? There was proof there were beings more powerful than the Borg right in front of us since "Farpoint..."
Q could have shown us ALL the other alien threats in the galaxy and none would be considered more of a possable threat than him.