I might be wrong here, but I never heard that PISA has proved that averages matter more than top results, I thought that's just the axiom they are basing their assessments on.

I understand why fostering top performants is a hard sell in a globalised society though. Poor countries will lose top talent to rich countries anyway, and rich countries can rely on the stream of talent from the outside. For the scientific progress of humanity as a whole, having geniuses matters more than rising average level IMO though