The Trans-Texas Corridor is not part of the mythical NAFTA Superhighway. There is, however, a real NAFTA highway being built: I-69. It will be an ordinary interstate outside of Texas, and part of the Trans-Texas Corridor system inside of Texas.
That is the problem. To upgrade in Texas to an ordinary Interstate is easy (most of the US-59/77 corridor is already 4 or more lanes, just need to build interchanges). But ego is getting in the way of reality...