I would hardly say that. We hardly ever actually see Palpitoad anymore, so I wouldn't count that one. Unfezant was sent to Oak's lab, so we don't see it anymore either. Ash is using Charizard again, and I would definitely call that a "powerhouse." Pignite is also pretty powerful, as is Leavanny when he uses it (again, not that often).
So the entire situation isn't as cut-and-dried as your question makes it sound. Yes, the three you mention are pretty powerful when he uses them, but they aren't the only Pokemon Ash has in Unova that can hold their own in battle.