The most convincing argument against expanding the NCAA men's and women's basketball tournaments from 68 teams is pretty simple.
Almost no one is asking for it.
There is no groundswell from fans. There are no massive ratings for the current First Four (i.e., play-in) games that suggest consumer demand. There have been no teams legitimately cheated out of a bid -- they had 30 regular-season games to make their case. There is no obvious competitive reason for it.
Outside of a few coaches and athletic directors, most of whom stand to earn healthy bonus money in having their teams reach March Madness, there isn't anyone speaking out in favor of this.