There aren’t going to be hordes of O'Rourke/Trump voters here, whether people like it or not. Trump winning TX by 2-6 is obviously far more likely than him winning TX by double digits or some nonsense like that.
There won't be hordes of O'Rourke/Trump voters but on the other hand it's important to keep in mind that 4% of O'Rourke voters voted for republican candidates for every other office (including for Dan Patrick), these voters could easily go to Trump.
By the way you have to take into account the fact that turnout among white non college whites should be substantially higher with Trump at the top of the ballot than without him, it should help Trump to run up his margins in East Texas
The Democrats have still not reached their ceiling with non-white voters in Texas, especially in the heavily Hispanic parts of the state where Beto ran behind/got fewer raw votes than Clinton. If they can engage these voters more in a presidential year, then that would offset much of the gains made by Republicans among non-college educated whites.