Big city mayor is no position to run for Governor (or anything else) of Texas from. Say his name recognition allows him to go from the usual 50-50 result in Bexar County to a 53-47 or whatever: that's not going to tip anything. The formula of 60+ in Travis, ~58% in Dallas, ~53% in Bexar, tying in Harris, and landsliding in the Valley and El Paso is a recipe for 43-44% of the vote and not a vote more. To get any closer than that in TX you need to cut into either the white suburbanites or the white rural voters, and there's no shot at either as long as Obama' president.
I think you're being a tad pessimistic. If a prominent Democrat with a Spanish surname and a decent record could rack up impressive numbers in the "del" in 2002, it's possible that the same results could be replicated amongst urban Latinos. The real slumbering demographic in Texas are the relatively new Mexican-American communities in Houston and Dallas. If Castro could achieve turnout at the same level as in 2012 and overperform Obama by 5-10%, he could get 45-46% of the vote.
In general though I agree with your post, the TDP is dead for the foreseeable future. Hope springs eternal on the internet. Reality says something different.
