Don't take this personally. Your comment just gave me an itch I had to scratch.

There are some words in people's explanations that always bother me. These are 'just', 'mere', merely' and the like. Why? Because they're not explaining, but explaining away. It's like saying: x is trivial, so it's not as important as you think.

But reality is much more complicated than that. For even simple things are hard to fathom for those not knowing how simple it is.

Now, back to the topic: no, medieval painters' lack of knowledge on how use perspective was not just matter of being stuck at conventions. When everyone is expecting to see a certain thing and you're payed to do that thing, it's not 'just' up to the painter see outside her/his perspective. What I'm saying is that takes a huge leap to break away from all that.

Beware of just explaining away phenomena.

To expand on this, Pirsig tears apart "just" as a completely useless word which reaffirms the speaker's belief but also trivializes the listener's inability to understand. "just" comes from ancient Latin "ius" and it means "I swear" or "I believe". When somebody says "I was just doing something", they are swearing a light oath that they were doing something.

Back in context, what the parent is saying is that the grandparent provided a "just-so" story: The parent finds it easy to believe that history happened in a way which neatly lines up with the story that they are telling. Did earlier humans know about perspective? Grandparent believes that it's obvious.

The word "just" in "it just took a reset in conventions" was intended as ironical (but it might not come across clearly in the text). A change of conventions is a large change that is no trivial matter to undertake.