I'm not a believer at all of the "what's good for Iowa is good for all the schools in the state" philosophy. I think it is usually the exact opposite as it impacts the ability to get in-state recruits, media attention, etc when Iowa has success. Iowa being great in WBB last year didn't help ISU WBB in any way last year or this year. It is far more likely to skew some younger, yet-to-be fans to the dark side, as well as potential recruits.Not to mention that in a 15 year span you'd have almost half the champions come from the state of Iowa. The state of Iowa would become viewed as a hotbed of whichever sport it is (football or basketball) and that would likely contribute to future success going forward. Hell, there's a decent chance in that kind of scenario one of the title games is Iowa vs Iowa State.
However, if ISU wins 3 championships in basketball or football and Iowa wins 4, I think the net gain is basically a wash or net advantage for ISU. People expect more from Iowa because of their media exposure, donor base, league revenue, etc. The Big 12 itself would benefit hugely from a member school having 3 championships.