I'm answering your question with a question because the difficulty of the evaluation process is the point. But there can be an evaluation, and there can be an objective answer for a set of values.
For instance, if we said "I would like a worldview that optimizes people not committing suicide" we could compare and contrast and say that worldview A is better than worldview B because A's adherents don't commit suicide and B's do. We can combine multiple factors, however imprecisely, and still compare A and B together as a rational person.
PG is making a similar argument in the "truth" of how art evokes subjective goods in humans. Humans across time and space are the measuring stick, and hence why the present is not overly weighted in his assessment of this evocative metric of art's quality.
PG's view on this is radical today but hasn't been for thousands of years and won't be again, because human nature doesn't change that much. Contextualization (acquired taste) can make one appreciate art better, but there is something transcendent across space and time that makes art lovely to humans, even lacking focused context.
