Well, less an afterthought and more just redundant content/lore really - they had far more relevance in the leadup to the shutdown of the original game (where their whole purpose was uniting the disparate Eorzean citystates together under one banner, citystates who didn't even have professional armies at that time) - with a giant space rock hovering over their heads like the Sword of Damocles and Nael van Darnus launching daily assaults against Eorzea... no wonder the GCs came about.

Post Calamity however, the GCs were kind of relegated to the background, what with more and more adventurers stepping up and filling in manpower holes in military affairs that that the GCs were meant to have filled themselves, and the Scions of the Seventh Dawn also taking the limelight (which is ironic, given the Scions predecessor in version 1.0 the Path of the Twelve actually started out important but then fell into obscurity when the Grand Companies were first formed).

Given how 3.1 and 3.2 have been hinting at, it's highly likely come 3.3 and beyond into the currently hypothetical second expansion, that the Grand Companies will undergo a revival and that will be when we see new ranks added, but until SE confirms or denies this, take this with a grain of salt.