I completely understand why people want to expect and presume donor fatigue will tamp down NIL. But I can tell you, the only concern of fatigue is with the small donor collectives. Big time donors are actually using it (not surprising) as a **** measuring contest and ego booster. They're used to giving millions a year to the AD, regardless of outcome. Now they can say, hey I paid X player directly to come here. And if you see tOSU who has spent upwards of $15mil this offseason on football players does well this year, that will hopefully get common folk to understand the true power of NIL.
High level donors care about ego, pride, and measurements far more than the ROI on something they've already been giving to for decades. There is NO sign or talk amongst anybody I've heard from (donors, NIL people, etc) that would suggest there is going to be any decline at the high end. If anything, places that rely so much more on collectives may have a disadvantage as those smaller transactions are from regular people that have disposable funds.
You can want something, you can expect something, and you can make a logical rationale for why something should occur.... but all evidence, talk, and discussions point to no high dollar donor fatigue anywhere in the near future.