50k made sense for the Bank because we rarely broke 50k average attendance (aided by visitor fans for big games/rivalries).
Even at rock bottom has attendance only dropped below 9k once, and with even moderate success, has been 11-12k+. I think 11-12k makes sense.
There was a Phase 3 in initial plans, which included an additional deck. The school subsequently decided against the Phase 3 additional seating, believing the 42k was more appropriate. No architectural malpractice involved...