Somehow they got an uptick in recruiting under Riley, then a big uptick early in Frost's tenure. The recruiting ground argument has never held up at all to fact or scrutiny. As you say, it took years of failure before their recruiting finally took a hit.
Maybe people forget that Nebraska, who went 3-6 in their league, hosted Oklahoma, who went 3-6 in their league and was completely embarrassed and didn't belong on the same field. But sure, they would've been better off in the Big 12.
When people push the "Big 10 has been bad for Nebraska" narrative, they are essentially saying Riley and Frost would've left the Big 10 west for the Big 12 and won more games. Which of course is completely idiotic.
They had a mediocre coach in the Big 12 in Callahan, and, wait for it, they weren't very good. They had a good coach in Pelini in the Big 10 and, wait for it, they were pretty good.
Pelini's winning% in his three full years as the Big 12 coach was just under 71%. His winning percentage as coach in the big 10 was just over...71%. Bill Callahan was a head coach in the Super Bowl and went 15-17 while in the freaking Big 12 North.