This sort of rejection is quite common. You can overcome the objection by submitting evidence that your mark has acquired distinctiveness through use. So there will be plenty of trademarks that seem generic/descriptive that have gone through that process.

edit: I originally thought that’s what was being rejected here. It’s not. The court only ruled that “OpenAI” is descriptive. Separately, OpenAI have also submitted evidence of acquired distinctiveness, which will be decided in due course by the trademark office.