I never said the thalaron weapon would certainly work against the borg. I said it had a good chance of working - of delaying or stopping the borg.
It's possible that the borg assimilated the weapon previously. It's possible that they will adapt in some way - although, between 7 of 9's reccomendation and the many opportunities the borg wasted, the risk of them developing grey goo is minimal.
But there is also a very good chance that the weapon will work.
What Santayana/Picard/La Forge did was give up without even trying - it's possible the weapon won't work, so we won't use it. A defeatist attitude - one justified by a moral argument with holls so big that the entire borg fleet could fly through.
What Santayana/Picard/La Forge did was start praying to some gods for deliverance - because they lacked the will and the creativity to even try to solve the problem themselves. That contradicts star trek's spirit - its fundamental humanism.
If the borg had the capability to create grey goo that can eat planets in minuts, they would have used that capability before "before dishonor".
Not really. The Borg don't anticipate new problems. They can only react to those problems and invent solutions as the need arises. There are any number of potentials the Borg had that they failed to utilize because the possibility never occurred to them. This is something we've known about them since the very beginning, so it shouldn't be that hard to recognize.
So you're admitting that the specs for the grey goo were not in the collective's data banks.