The decision is the correct one. Ash had a rough go of it against the Jayhawks, completing just 8-of-16 passes and throwing two interceptions. He even struggled to complete screen passes on multiple occasions. But that doesn't take away from the fact that Ash won the starting job in the first place, or that he's proven he can get the job done.
It's also important to note that the offense has been at its best when Ash has been firing well. Texas put up 66 points on Ole Miss because Ash threw well. The Longhorns came back to beat Oklahoma State because Ash didn't fold under the pressure. And Ash averaged 14.4 yards per completion in a shoot-out win over Baylor. And even with forgettable performances against Oklahoma (he didn't have much help here) and against the Jayhawks, Ash has a quarterback rating of 150.3, certainly good enough for the Longhorns to put up points offensively.
If nothing else, I feel like Texas has discovered the best role for McCoy in recent weeks. It's no secret McCoy is a "rah-rah guy." And that's the kind of guy you want coming off the bench in a crucial situation. He watches film, prepares well and can give the team that added "oomph" when he enters a ballgame.
Ash doesn't have that. Ash's best asset is his skill set and his upside. So he's not as ideal coming off the bench to provide punch. In fact, if you even look back to 2010, there isn't one instance of Ash coming off the bench to provide added pop, whereas McCoy did that against BYU, against Kansas State and obviously last week.
So in picking Ash as a starter, Texas essentially made two correct decisions. First, that Ash is the right man for the starting job. And second, that McCoy is the right man for the off-the-bench guy. And both give Texas the best chance to win.