My opinion on this issue is drawn from my understanding of the history of the subject. It was, unlike other classes, designed as a regional class only. It was a place to run your old SS car when it was too old to run nationally, and you either didn't want to buy a new car to compete in the "car of the year" class or you thought it would be a cool idea to modify what you had to make it faster. Later, the opportunity to move from IT to Production was offered as several of the National Production classes were dying from poor subscription. That process, IMHO is a good one.
The only exception to that was American Sedan. That was a class developed, like the ITR concept, for people that had a need for more speed. It existed for a couple of years as a regional only class built under the standards as those for all of IT. The problem was that the cars were too fast for stock brakes and suspensions. Since the old A Sedan was now a part of GT1, there was an opening, and by going to National status, the rules could be modified for safety and performance. My buddy that drives AS told me once that he was glad to see the class go national, until he found out how much it cost. His is no cheap effort. He's sat on the pole of the Runoffs more than once and finished on the podium a couple of times. He no longer runs Regionals as his emphasis and money are now focused on National competition.
Even though the national ranks are full of old cars, my sense is that the emphasis in National racing is toward the newer, flashier cars. I won't say that the direction for National racing is towards a professional series, as others have, but it was only one president back that it seemed like that was where it was headed.
Spec Miata was embraced by National racing so that it wouldn't get left out in the cold. I don't believe anyone ever thought that it would take off like it did, but I feel that Mazda is pretty tickled about it. Even though the cars aren't all alike, to the average spectator eye, they are. SRF puts on just as good a race, but you can't drive one home.
There is a chance that ITR might go National...maybe ITS and ITA, too. But I doubt it. People talk about the price of racing going up. It won't. But if you want to run your ITA Miata at the pointy end of a national class, it will.
My question is, why? Why would you want to change theaters when the play is doing great where it's at? Having the national office and it's rules makers and changers put us at the end of the supper table isn't all that bad a deal.