Well Mike, your lawyers might want to contemplate the definition of the word "redundant", which according to the dictionary is:
"not or no longer needed or useful; superfluous"
I think they will have a hard time convincing anyone, including the ACCC, that your past customers who paid for this software are now finding it "redundant"........(no longer needed). As to your claim that you can no longer support this software, well I don't think any of us need or expect you to do this, so that's just a nonsense statement. We simply demand that you not render it unusable.