> There still are some polymaths, but it's hard for them to make as fundamental a contribution in fields that have already existed for a long(ish) time.

Yes, but it misses the fact that polymaths historically have solved that problem by creating new fields. Richard P. Feynman, as one example, lectured on nanomaterials and nanodevices decades before the technology existed to make his ideas practical. Einstein shaped relativity theory about four decades before there was any way to confirm (in detail) its theses or apply it to practical problems.