not to be a shill, but isn't it good for the non-profit to own a big piece of a successful company?

I think it depends on context.

If the private subsidiary was doing semi-unrelated stuff to the goals of the non-profit, and using it to fund the non-profit, then your logic could make sense - for example if a cancer research charity owned a profitable business and funnelled the profits up to spend on research, great.

But in OpenAI's case, the claimed goals of the non-profit were essentially "do AI in a way that puts safety above profits". And whether or not one agrees with their previous approach to safety, or even whether safety needs to be cared about, it's undeniable that the for-profit business isn't acting as useful fundraising for the non-profit's goals, it's literally acting in the opposite direction.