"Under accepted" implies not enough acceptances to fill available seats. To your knowledge, do any schools currently have open seats? Why is it not possible that less acceptances across the board will result in nothing more than everyone's yield going up, and overall WL movement going down, notwithstanding the fact that lots of candidates that might have been accepted (or rejected, for that matter) in previous years have been placed on a WL this year to provide a needed buffer due to schools' lack of visibility this year? It seems to me that less acceptances has to result in less movement, not more, or even the same, as compared to previous years.
All of this is likely to result in less optimal outcomes than the alternative under the MAR, since PTEs will "settle" for what they have, since they won't be called off the WL of preferred alternatives, because people won't be giving up seats at those alternatives, because they are not being called off WLs from their preferred alternatives. If you don't think people holding multiple acceptances is a widespread thing, and most schools have "under accepted," what vacancies are going to be created when CTE time rolls around to incite significant WL movement? It seems as though fewer acceptances will mean nothing more than less multiple acceptances, less optimal outcomes and less WL movement as compared to prior years.
What am I failing to consider?