Great thread with lots a good input.
I have been following Ohio HS football for a long time now and I can tell you it certainly appears that the talent and elite teams have dropped off some but much like many things there are many aspects to this.
From the 1916 (Toledo Scott) through 1961 (Massillon Washington) Ohio was a player on the national scene when it came to HS football. There are several polls that have ranked HS football from 1910 going forward (some have gone, new ones have appeared). From 1961 to 1976 (Moeller) Ohio did not have a national poll champion. In the late 1970's and 1980's Moeller dominated the national scene winning some form of a national poll championship in 76', 77', 79', 80', and 82'. In 89', 93', and 95' Ignatius carried that banner and was ranked tops nationally followed by McKinley in 97'. The only Ohio team to be tabbed as a national poll champ since would be the 2007 St. Xavier team. You can argue that these polls are meaningless but they carry weight on what Ohio was thought of on a national level. I'd also argue Colerain of 2004 should be listed but regardless it is 2019 and Ohio has not had a top ranked team in 12 years and only one in 22 years. From 1976 to 1997, a 21 year stretch, Ohio had 9 national poll champs. Not to mention that in that time-frame Ohio would routinely have multiple teams ranked in the national polls whereas today, as a state, we are lucky to have one.
With all this said I would put Ohio's HS hey-day as being from 1976 to roughly 2004. Not only did Ohio dominate on the national level team wise, but the state also produced some of the nations top talent (3 Heisman trophy winners). Not only top talent but also depth in talent routinley peppering DI college rosters.
The #1 reason IMO would be population decline.
Cleveland
Cincinnati
Everyone understands Ohio's troubles in terms of population recently and simply using Ohio's two traditional metro cities you see the population decline. Columbus would be the lone exception but Toledo, Dayton, Akron, Youngstown, and Lima all look similar in terms of decline. From 1960 to 1980 much of the population lost was to the suburbs. From 1980 to today much of that brain drain was to other states. Look at what has happened to Youngstown or even my favorite poster child for Ohio manufacturing decline Fostoria, a city which not long ago (1991 & 1996) won two DII state championships. Fostoria has a long history of football prowess not only with the public school but also the Catholic school St. Wendelin. Fast forward to today and Fostoria is a shell of its former self. The Redmen are now a DV school and St. Wendelin has shut its doors.
Further, my freshman year at one of the local Catholic powers we had 80 kids on the freshman squad. I'll never forget that we had a couple kids who never saw action all year (and their parents did not bi$tch - different times). Today that same school has 25-30 and the demographics are far different. A good chunk of my HS friends and teammates are raising their kids in places like Dallas, Kansas City, Atlanta area, Phoenix, etc...
Other reasons (previously mentioned) would be school size and specialization schools.
As already mentioned Allen Texas has roughly 6,500 students. I have a friend in the district and these school districts keep their high schools this big for one reason: football. However, specialization and recruiting have changed the landscape. IMG was essentially created to be a super prep recruiting football team in 2010. Others have followed suite. Today you have "all-star" 7 on 7 teams and I have even begun to see "all-star premier elite" pee-wee and jr. high teams start to pop up. So the same BS that bottlenecked baseball talent is happening in football.
With all of this said, keep in mind we all get fixated on BIG. Yes, Ohio does not have a USAToday poll champ or is no longer sending the elite talent to college like it once did but football in Ohio is still a really big deal. Our body of work across the board and through the smaller school divisions is as good as anyone else IMO. I beg anyone to go watch football in MI or IN. It is simply not the same. Even FL, which produces top talent year in and year out, does not have the following for HS football like Ohio.