If you can sue someone for using stuff you copyrighted to train a model, "Open"AI won't be a thing in the first place. Moreover, "Open"AI doesn't have copyright on the outputs, only the terms of services stops the receiver of the output from developing competing products. If you received the subsequent product, there is nothing OpenAI can do to you because you didn't enter into an agreement with them in the first place. None of this involves any copyright.