If this rumor is credible, I understand why Disney would have to change the ride. I just don't think it's a good reason. Test Track is a terrific piece of Imagineering, and it is popular with WDW guests. Those are two reasons that should justify the ride's continued existence in its current form at this time. A good refurb, some small show enhancements, some new post-show exhibits, and an upgrade to the Radiator Springs Racers version of the ride vehicles would be welcome, but, at its core, it should remain the same ride for now. GM may be putting up the money to do it, but Disney would still be using time and creative resources that would be better spent on some other area that actually needs an improvement in WDW.While I partially agree with you, your not considering that Test Track is a sponsored pavilion, and this money is not coming from Disney, rather its corporate sponsor GM.
That's the downside of having a ride sponsored, I guess.