If you chose you use your resources to upgrade every character you have, it’s only your fault your GP is inflated. Even if you didn’t know about GA in the future, spreading your resources so thinly is something that’s ill advised. For TB for filling platoons, you can have Level 1 7 star characters to fill them if you need to, so I don’t see that as an excuse. For TW Extra fluff doesn’t really help. For GA they’re in a hindrance.
Personally I limit myself to a few characters for gear grinding at a time. So I have a lot of characters sitting at 7 stars waiting until I can gear the whole team together. That’s how you grind a team with synergy and not shoot yourself in the foot with leveled up toons you cannot feasibly use.