We're also talking about an effect that literally was made for a trophy, not a reasonable goal ever meant to be obtained by the masses nor meant to be a true beneficial upgrade. The reason it makes you glow in the first place is because it's a trophy to make you stand out, I myself plan to make an Afterglow for my Murgleis provided I ever finish it, that being said though it is simply not an upgrade meant to make you actually more viable in combat, and in all honesty, I am thankful for that. Afterglow is not meant to have effects that make them stand out, Afterglows are meant to be a way to show one's devotion to a job.
I have little doubt in my mind you are one of the best Beastmasters in this game, that being said, I assume you made the Afterglow knowing what it entailed, you went that extra mile and as a result you glow, you have a special visual effect on your character that few ever have or ever will in this game. That itself, is the reason for it to be a worthwhile goal, while to many people including myself it is not a goal that would seem worth pursuing, some do think it as such. The thing is, these goals are simply outrageous for the same reason you mentioned, the 2 billion gil price tag, perhaps it seems wrong of me to say it should stay as it is, but in all honesty, I feel as though it has a purpose to serve as it is right now, and it fills that position well, giving it massive benefits for upgrading to that final level seems like a bad idea to me, sorry.