Consider the opposite prevailing, where I can legally protect my warez site simply by saying "sorry, the conversation where I sent them a copy of a Disney movie was private".
The legal situation you describe is a matter of impossibility and unrelated to the OpenAI case.
In the case of a warez site they would never have logged such a "conversation" to begin with. So if the court requested that they produce all such communications the warez site would simply declare that as, "Impossibility of Performance".
In the case of OpenAI the courts are demanding that they preserve all future communications from all their end users—regardless of whether or not those end users are parties (or even relevant) to the case. The court is literally demanding that they re-engineer their product to record all communications where none existed previously.
I'm not a lawyer but that seems like it would violate FRCP 26(b)(1) which covers "proportionality". Meaning: The effort required to record the evidence is not proportional relative to the value of the information sought.
Also—generally speaking—courts recognize that a party is not required to create new documents or re-engineer systems to satisfy a discovery request. Yet that is exactly what the court has requested of OpenAI.
If specific users are violating the law, then a court can and should order their data to be retained.