I don't expect this to be a popular statement or to get a book deal but....
I have always been told "when you pull with the NATPA, you pull with the best".
The point you make has validity. When bigger engines enter Div4 smaller drop out or move to Div3. 3500# Div4 is a great example considering the arrival of a large H. Now there are 3-5 contestants. I wouldn't hear of limiting this H even to be "fair to as many people as possible". Oliver 77 have fell prey to this years ago because they were tough. Similar situations have followed with domination by M Farmall series in 4500#.
MM and Case are only bigger in cubic potential. It took until now for guys to use it in U series and pull frequently at the national level. Now it is called unfair to too many people.
This year we have a new group of pullers consistently winning, just as the last group of consistent winners came to be. Many guys were tired of being the big fish at their local pulls and wanted to pull with the best. So they tried and won, but it's no longer "fair" for a few old big fish.
Div4 hooks COULD fall, yes, even though I've only seen numbers grow dramatically. I am throwing my own MM into the 4500# mix because I can. I may even win. My engine will not go in a 5-star for fear of rule changes. But I sure as hell want to pull with the toughest ones out there, not necessarily the biggest group.
As they say, if you can't run with the big dogs, stay on the porch. I don't care to be a big fish in a pond of minnows by kicking out all the Pike. Nor do I care how many people I "awe". No big cube tractor's presence ever made my hook shorter than it would have otherwise been.
The NATPA banner reads "pull with the best", not "pull with the most". Numbers are great, but if you rule out the cream of the crop, you'll never know. That gives meaning to that shiny platic trophy.