I don't agree with the "MK is for Kids" statement but there is something to this. I used to roll my eyes at the Speedway - until we took my son to MK for the first time and I saw his face as he got behind the wheel. And we'd been on other "car on a track rides" at other local amusement parks - but this one, for some reason, was magical for him. So much so that on our next visit to MK later in the week, we rope dropped the speedway at his request. You know your a parent when you rope drop the Tomorrowland Speedway.
I also read somewhere (probably here) that the idea of removing the speedway was surveyed and scored low. Apparently there is a nostalgia factor, and many parents who went on the speedway as a kid with their parents dream of taking their children on it. Kind of a rite of passage. So I can see why there is a desire to keep this attraction, whereas if you asked me about this a decade ago, I would have been right there advocating for taking it down for a better attraction.
All that said, it does need a reimagining. I believe the current rumor is that it will be rethemed with Tron styling and changed to electric cars.