That's also the proof that they could have given PLD approximately the same ability to counter knockback, but in a much more effective it.
The tricky part is that Plunge needs a target, so, each time they would like to negate "Tempered Will", they could simply make you unable to select a target, instead of just bypassing the effect.
They could replace Tempered Will with the gap closer from the 1st boss of the Vault, Shield Bash included.
