I agree, NASCAR is going to show us her real abilities (or lack thereof) as a driver. I do not think it's fair to give her a couple races and make a decision there. But her ability to adapt to these cars I think will say a lot about her talent as a racer. Even Franchitti had a pole in Nationwide last year and had some decent runs in nationwide and arca. But his time there was considered a failure. Danica will probably get more time in the cars than Dario thanks to her sponsors, but at a certain point next season if she's not running or qualifying mid pack with top equipment, people are going to question her decision. And that can't be good for the brand. I suppose it's a risk she's willing to take, and she's never lacked for a healthy ego.
I'll also add that while I don't think her absence from the IRL will effect ratings all that much, I DO think it will have a significant effect on the IRL's ability to get a title sponsor. She's the face for a big affiliate sponsor, Peak Antifreeze, and my guess is that a "star" leaving the series will not look good in IRL marketing/sponsorship meetings. She adds a lot of perceived value to the series, even if it's more perception than reality.