theres no reason to replace anything. And it eats a decent amount of guests. They need to use some of the empty spaces they have and lots that have never been used. Worst case push out the boundaries. Disney shouldnt even be thinking of replacing at this point unless its to plus a ride thats already there.
In most cases I'd completely agree with you but the Speedway is an ancient relic that actually makes the park worse (the sound, the smell, etc.) and kids can experience a better version of it in numerous other places. It brings essentially nothing to the park beyond capacity, and they could easily increase the capacity with two new attractions. Even just changing it to electric cars would help, but they could do something so much better with that space.
There is no additional land to build something in Tomorrowland (beyond the currently empty buildings which need to be filled, but they can't build any major attractions in those spaces like they could on the Speedway plot) now that TRON is going in.