The whitepaper says that +2/3 of the entire validator set (in voting power) must sign every block. In the ideal scenario (in other words, in the stable state) every validator does sign every block.
Yes, it is pointless for me to argue about an undefined system. If you mean to say that 67% of the validator set must sign every block, then you shouldn't argue with the premise that 100% must sign every block.
It would be interesting to see a more well defined attack from you.
I've explained stake grinding attacks. Your response was redefining the system opening a new attack vector.