The Girl With the Pearl Earring is considered a masterpiece because of the technological limitations of the time.

Blue was one of the most expensive colours because the ultramarine dye was derived from lapus lazuli, a rock imported from Afghanistan and ground with a labour-intensive process. Medieval European art typically depicted the Virgin Mary in blue. The expense indicated devotion.

Someone living in that time period would know anything in ultramarine is important.

Except Vermeer used it for whatever he wanted, including a blue turban on The Girl With the Pearl Earring (originally called Girl with a Turban). The pearl is expensive in the world of the painting, but the blue turban was expensive to create in real life. That is the central mystery of the painting.

But we literally cannot appreciate that because we did not grow up in a world where ultramarine blue was as expensive as gold, because synthetic ultramarine was invented in 1826. That's why you care about visual interest and aesthetics instead of reacting with "Holy shit! Why is this blue?"

Our descendants will likely feel the same about the art we create today, and ignore whatever aspects of it are trivialized by AI.

I see what you mean, but I don't think this is super accurate. There are similarly large (and larger) patches of blue in many paintings by Vermeer and others from the Dutch Golden Age. Ultramarine was as expensive as you, but it was demonstrably used in many paintings from the Renaissance at large. The historically expensive blue paint is not the primary thing people think about when considering this painting, nor is it the reason this painting is uniquely loved among paintings of the period.

Of course, the expensive paint is a part of the history of the period, and paintings like this one become a symbol of the period as a whole. Appreciation for the period is certainly part of the appreciation for the painting.