It's not just NIL, it's the transfer portal that goes with it. The major programs scout the mid-majors more than they scout HS kids. The days of Loyola of Chicago making a run are over. The players they had to make that run will get pilfered away before they get to that point.
IMO, this is the beginning of the end of what makes the tournament special. Maybe there are a lot of people out there who are happy to see all 4 #1 seeds make it to the end. For me it's just very uninteresting. As you said there were very few upsets, very few nail biting games at the end, no Cinderella team at all. Boring. Making brackets, which seemingly the whole country does, is boring if you can just mark down the favorites and be right most of the time. It will take a couple years to be noticeable, but if we have a couple years in a row like this you'll see decreased interest. Jacking up ticket prices at the sites didn't help either. I noticed a LOT of empty seats in most of the early round games regardless of who was playing or where it was played.