This is the way the game is going. If they have plans to bring Carbuncle back to the forefront, they will. If they don't, then they won't. Mechanically, there is nothing the carbuncle duo brings to the table that the egis and fairies don't either do better or have a completely different purpose.
The only way you're getting the carbuncles back is if they allow reskinning the egis.
I also have to wonder what exactly your fixation is with the carbuncles. They're just a pair of radioactive rodents that serve as your "training" summons until you get better ones or, in the case of SCH, your correct ones.
It's almost disturbing how much you're fixating on these.