I really like Beto, but he'd be better served as a VP to a progressive like Bernie.
and for running against Cornyn, I doubt he;d win that either. Cornyn is by far less hated, and generally more accepted by the educated / center republicans in the state. Beto would absolutely lose the few red counties he picked up, and I think beto would lose by even more than he did Cruz. So, I cant fault him for this.