I don't think this is complete fantasy, maybe a bit too dem friendly
I have definitely thought that a perfect storm Dem candidate could eek out a win over Cruz.
However, I was probably incorrectly worried that Beto would not be that candidate (I thought Rep Castro had better shot)... But it appears Beto may indeed be that perfect storm- Young, Charismatic, Tireless Campaigner, Believable/ Relatable, Able to build a groundswell of support, etc.
If you look at the trends in TX over past 6 years, non-hispanic whites have been quickly moving toward a minority (texas will soon be a minority majority state). And Beto is probably going to be able to get a much larger % of youth vote & minorities vs Cruz (than Hillary vs Trump).
If a ground swell is indeed building for Beto... especially among Under-40s... there could be enough energy and turn-out (among typically lower turnout voters... youth vote & latinos)... to push Beto to a narrow win.