Texas had been pretty mediocre for a few years before BYU beat them
They'd gone 22-16 (including 11-15 in conference) in the three seasons before, which was their worst 3 season stretch since...well, since they'd been part of the Big 12. They'd finished 6th, 6th and 3rd in their conference in the previous three seasons (actually got up to #2 in 2013 when they first lost to BYU) and just came off a loss to a new TCU. Make no mistake, this was no longer a good team.
But, being Texas, the expectation was that they were just around the corner, that they didn't need to change things to improve, that order would just reassert itself. BYU didn't start their downfall, but they exposed it for what it was, and made it so fans couldn't just pretend Texas' slide wasn't happening in front of their eyes.
Not a pleasant spot for Texas, but honestly, they needed the awakening. Clearly they still aren't back yet, but at least it forced them to address the problem. Honestly, they've had more success under Tom Herman than the last 5 years of Mack Brown or any point under Charlie Strong, so they're building in the right direction.