KUs stadium capacity is fine for staying in the Big 12, it's absolutely not fine if they're trying to be attractive to the Big 10. Pointing out that Northwestern also has a small stadium isn't a winning argument. The Big 10 isn't going to jump through hoops to add more Northwestern.
UNC dropped their stadium attendance from 62k to 50k and still remain one of the top targets for expansion.
And sure, Northwestern is an extreme example of capacity reduction, but it won’t matter if KU stays at 47k or drops to the low 40’s, they have to do something to fill the stadium consistently and add premium seating. And KU’s BIG10 invite would come due to the basketball program, not because of the football program having a 50k seat stadium.