I think it would be fair to present the other side of the argument.
Yes, certain conferences have adopted this model and it may work best for those schools. What people overlook is that for every Ohio State or Texas there are states like Kansas, Iowa, Mississippi and even Connecticut, many of which are low in population, many without professional sports inside those states, where the state program becomes the primary focus of the sports fans in those states.
Yet you look at each of those schools, they have the passion of being "The Team" for the state. Kansas and Iowa, although small states, have a passion for their teams (Kansas basketball and Iowa football). They support their teams with fervor. Same with Ole Miss and UConn. Does Pittsburgh really support Pitt? Does BC? Even GT is an somewhat of an afterthought in Atlanta. Secondary schools have to be in the public eye to be relevant in their state, flagships don't.
However, in the higher-populated areas, it is not necessary to have a state flagship program to be part of the conference, although some are desired. UMass, New Hampshire, Montana and Rhode Island are certainly state flagships but their sports programs, for various reasons, are not capable of competing at the highest level. Connecticut rose to prominence through their two basketball programs but it took until their membership of the Big East in order to use the competition to better themselves. The Big East, just like the ACC, had a mix of state, city and private schools. What broke them apart were the schools were unable to reconcile their differences. The ACC simply took advantage of the opportunity presented.
You forgot North Dakota, South Dakota, Idaho, New Mexico, Maine, Vermont, and Delaware. There is a reason that these schools are not in P5 conferences (or even FBS) because not all state flagships schools are fit to be in a major conference. Not all want to be, or just plain can't. The thing is about schools in higher populated areas, is the competition. Pro sports dominate many areas and the secondary schools in higher populated areas have to compete with much more than the flagship. The trials of college teams in NY have long been discussed. Some areas are just pro focused. Boston is a pro town. They love their Celtics, Patriots and even their Bruins. BC has to compete with those for loyalty. Loyalty is one of the greatest attributes in a fan base. Guess who makes the most loyal fans? Alumni. I still follow my alma mater and they are DII in everything but hockey. Flagship schools have a tremendous amount of loyal alumni, more so than many private schools.
Yet, I really think it's not about flagship, but flagship like. I consider VPI like a flagship school. It's not, but they have the passion and the size that a flagship school possesses. Michigan State, Auburn, Clemson, NCST, and even Louisville have the same attributes. Large, loyal alumni bases. States like Florida, Texas, and California can have multiple "flagship" schools (and do), even though only one might be labeled as such.
That other problem with going with a state flagship model is that not all state schools have the willingness to expend resources to compete, either because of choice or economic challenges. Until this year, the Mississippi schools have been laboring in obscurity because of the difficulty of competing with the Alabama schools, LSU, Florida and Georgia. It is a very poor state, so the people there can't spend like people in New York would for sports. Rutgers and Maryland have had to deal with pro sports right in their backyards, something that Nebraska never had to worry about. Even then, UNL (and RU and UMD) had to look to the Big Ten to ensure their relevance in the new college sports landscape.
Yet, The Mississippi schools HAVE become relevant (I won't say how I think they did become relevant). They have the resources to pay for top notch coaching staffs and facilities. Will Miss State keep Mullens after this year? My guess is no, but they held on to him for 5 years. Not every school is going to compete in every sport. Yet, if they have a large, strong alumni base, they will be an asset to any conference they are in. Rutgers was a risk. They don't have the athletic tradition that other large, public schools have. Yet, with their state population and large enrollment, they already are an asset. Smaller, private schools don't have that luxury.
What the ACC has is their position to access tens of millions of more people than conferences like the Big Ten or Big 12. That doesn't mean we should overlook programs that would improve our athletic standing. That's why I'm in favor of UConn. I only wish more within the ACC see this as I do.
The ACC may have access to all of those people, but how many truly want that access? I give you NC and Virginia completely. But every other state, the schools there are all shared or overshadowed by larger, more prominent schools (or in BC's case, pro sports). Florida has UF, but FSU does take a good share. Clemson has a fair amount of clout in South Carolina, but plays second fiddle to USC in a very small state. Georgia Tech is secondary to Georgia even in Tech's hometown of Atlanta (where they have to compete with The Falcons and Hawks add well). Pitt? Again, they have to compete with their beloved Steelers. 'Cuse? They have upstate NY in spades. I guess they register in NYC, but still plays second fiddle to all of the pro teams.
The ACC did what they had to do. My guess is that given the choice, The ACC would be built like the other conferences. They didn't have that choice and went the only direction they could.