I don't agree.
The Texas-OU rivalry is one that transcends the condition of either program. The hatred has been there for many, many decades, and it doesn't fade just because one or both programs hits a slump. Its roots also run deeper than athletics -- think border-state animosity. There's also the fact that Texas and OU have historically fought over the same talent in recruiting much more than OU and Nebraska ever have. OU and Nebraska really became rivals only because both were competing for conference supremacy and national championships in the 1970s and 1980s. If both programs hadn't been elite and prominently in each other's way during that time, there would be no such thing as an OU-Nebraska rivalry.
Certainly the games that OU and Nebraska played in the mid- and late 1980s were bigger and more consequential games, but that doesn't necessarily entail a reordering of the rivalry hierarchy. During the early years of the 2000s, when Texas fans expected to beat aggy's ass every year but got cold sweats before every game against Leach's Air Raid, no one would seriously argue that Tech became a bigger in-state rival than aggy. Anyone who made that claim just wanted to needle an aggy fan. Even when OU hit a slump in the 1960s and our games against Arkansas were our most nationally consequential, Royal knew that OU was our chief rival. That never changed.
Ask Barry Switzer -- the OU coach during the majority of OU and Nebraska's overlap as simultaneously elite programs -- who Oklahoma's main rival has always been. He's most definitely not going to say Nebraska. When Switzer was feeling the heat from OU fans in 1983, it wasn't because he had dropped three straight to the Huskers. Switzer has always said that Texas-OU was the game that he had to win to "feed the monster."