Agree. But it's worth noting that they already have a bit of a hedge in the description for private mode:
> This chat won't appear in history, use or update ChatGPT's memory, or be used to train our models. For safety purposes, we may keep a copy of this chat for up to 30 days.
The "may keep a copy" is doing a lot of work in that sentence.
"for up to 30 days" though. If they are being kept in perpetuity always they should update the copy to say "This chat won't appear in your history, we retain a copy indefinitely"