Quite frankly I would prefer new highways built closer to me than I-35 is. Corpus Christi, the city I live in is just about due south of Dallas, but I currently have to go to as far West as San Antonio if I want to drive to Dallas on Interstate grade highways. Having a highway east of and parallel to I-35 would improve north-south traffic for people living on the Gulf Coast and the Rio Grande Valley.
Anyway, it isn't just a highway right of way. There will be a double tracked railroad right of way too. Most of Texas has really lousy single tracked rail connections like the ones connecting Dallas, Houston, and San Antonio (three of the ten largest cities in the US). Texas today has a 21st century population of over 20 million people but has a rail capacity that isn't any better than it had in the late 19th century. It makes sense to build a few concentrated multipurpose rights of way than to build a hodgepodge of poorly connected ones.