Why hasn't Paizo done more to balance these low-tier classes? Don't get me wrong, new classes are great, but I'd like to see balance changes too.
Rather than answering that in another comment, I'm turning the question over to the community: What reasons has Paizo had for not changing the core classes' tiers?
The Pathfinder CRB provides 11 classes: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, and Wizard. When played vanilla, these classes are T4, T3, T1, T1, T4, T6, T4, T4, T5, T2, and T1, respectively. When played with all Pathfinder published material, the tier of each class is roughly the same, maybe allowing a one-tier change.
It does seem deliberate, since additional material affects the power dynamics between classes so little, and since whenever additional material does have the potential to alter balance, it gets errata in such a manner as to ensure that doesn't happen. If it were entirely due to neglecting balance considerations entirely, one would expect additional material to correct or reverse the errors by way of random chance, if nothing else. What reasons do the Pathfinder developers have for maintaining the original power discrepancies between classes?
Answers from developer quotations are preferred, but experience working for or with Paizo might also work. As usual, remember to "Back it Up", since speculation is not an acceptable answer to SE questions.