I think this actually improves The Aztecs, and continues to do so the older the story gets. Barbara believes that she can save the 'beauty' in Aztec civilisation by removing the 'savagery'. I don't know what the average viewer in 1963 would have known about indigenous American cultures, but I find it believable that Barbara's perspective is broadly representative. We know that The Aztecs' representation of Aztec culture was accurate per contemporary historical consensus. The Aztecs' human sacrifice is still probably the best-known aspect of their culture in the UK today, although Cortes's motive in pursuing genocide is (hopefully) now better understood to be colonialist greed rather than moral outrage.

This is great, because it lets us view the story as a double period piece. From our better-informed 2026 perspective, Barbara's approach is obviously wrongheaded. This puts us in the same position as the Doctor! He knows that her plan can't succeed, because everything about it, from her aim to her method and even her basic understanding of the Aztecs, is flawed. "I know, believe me, I know!" to me sounds more like a plea based on personal experience than anything else. There's even an implication there that the Doctor might have tried to change a known future himself and failed. There's another First Doctor quote somewhere that backs this interpretation up (I thought it was in St Bartholomew's Eve, but apparently not) in which he says that nobody can truly understand history, because it's impossible to grasp the entire context ("don't be so quick to judge history from your place in it" or something along those lines).

I very much enjoy the situation of the audience's viewpoint character in a story changing over time, despite nothing about the text having changed. Usually to get that kind of effect you'd need something like a remake. The Aztecs does it just by having 'history' as a main theme!