I think the headband of intellect is meant for rogue and fights, so they can take the eldritch knight and the arcane trickster and be as good as the wizards.
You are right that to balance things out they should add objects that help all the classes. Not just +1 to hit, but also magical robes (I found just 2), more belts, objects to set other attributes to 18, etc.
But balancing loot is complicated. Right now I think rogue have it best. Even the magical weapon you craft is designed for them.
As for fighters, most rare weapons are two handed. So fighters who want to be duelist, have worse options. And clerics are severly handicaped.
