my personal opinion is that i don't see Autopia going anywhere. It is a staple, many amusement parks around the country have the same type of attraction. Yes, the cars go slow, your on a track, etc...But for many kids, it is their first tast of driving. For me it is such a nostalgic ride that takes me back to when i was my children's age i was just as excited to drive the cars then as they are now. I don't see Disney ever eliminating a ride that accomplishes so much. I would say that the fact that it is a popular attraction is testament to that.
Regarding the forthcoming Cars attraction at DCA, i don't see that detracting from Autopia one bit. It is in a seperate park and it will be a totally different type of attraction. It will utilize technology in the Test Track ride in Epcot. I don't see it bearing any resemblence to Autopia and thus not in competition.