Here is why Mount Carmel will not petition up to 8A at least for the next few years in my opinion. They right now are within one state title of the all-time record just behind Joliet Catholic. With the parity in 4A-6A, Mount Carmel knows that if they stay in 7A they can expect to dominate that class for the next decade and potentially have a dominant run here of 3, 4, 5, 6 etc. straight state titles and eventually overtake Joliet Catholic. That makes them the most successful program statistically when it comes to IHSA state championships and that is a powerful recruiting too if a student wants to play football and attend the school for other academic reasons as well.
Interestingly enough this is also the reason why Joliet Catholic will not petition up above 5A. They don't want to play East St. Louis who plays a semi-national schedule in 6A, Mt. Carmel in 7A, and Loyola and the rest of the public schools in 8A.
In addition, Mt. Carmel plays Loyola each year. Loyola is solidly in 8A. So if Mt. Carmel can beat Loyola each year they can claim that they are the best team in the state as long as Loyola also wins 8A. And even if they lose to Loyola and still win 7A they can make the argument that they still are better than everyone else minus Loyola.
The way I see it is Loyola is never coming down from 8A, and East St. Louis for the time being will never be allowed to move up from 6A. Those are two nationally ranked Top 100 programs most years so the other schools that need to rely on fluctuating enrollment each year are not going to take a chance of running into a roadblock in those classes if given a choice.
At the end of the day as they say "Banners Fly Forever" so honestly how many players are going to opt not to attend Mount Carmel or any other Catholic School in 5A-1A simply because they want the higher class 6A-8A state championship?
This is why they will stay where they are at unless other circumstances imposed on them would threaten their success model. Only then would they maybe reconsider their stance.
Look at is as the Illinois High School Football equivalent of Notre Dame and the 12 team playoff. Why join a conference and play tougher competition each week if your brand name alone gets you into the new playoff structure with a 10-2 record each year? Why play up a class when you have an easier path to a championship?
You may not like it, but at the end of the day for these schools who are guaranteed zero students every year it is the best formula to ensure continued athletic success and more importantly the ability to stay open each and every year.
But long-term I do see a scenario where the vast majority of the IHSA being public schools forces a situation that makes some of these schools play up in class against their wishes or separates them out into a private class with their own separate playoff structure.