I kind of understand that frustration. Writers have to be careful what end goals they set up from the beginning. It could have been written with better care. Ultimately for me, that was just a writer’s mistake and the journey mattered more than the destination. I’m just glad Deku is happy with his outcome and don’t need him to be massively successful by worldly standards.
Dreams/goals can evolve and change. I do think this change sort of makes sense for a person who grows up and matures. Besides “greatest” is still a vague term. In the end that goal became less important and that’s okay. Many people don’t fully achieve the same goals they had as kids because it changes. Deku even mentioned in the last chapter/afterward how he enjoyed having the teaching role more than simply pro hero role.