A question about reffering to source

In addition to the absence of original documents, at what point is the work of later numismatists more valuable than the original historical sources?

If they contain more information and fewer errors?

CAN YOU OFFER AN EXAMPLE？

I think it is the same as in the sciences.

When there is new data, or a new theoretical framework to explain the existing data which work better than the earlier one, then the new theory will eventually become the accepted one.

A large portion of early numismatic works are text only. A later work with images is more valuable when it clearly shows differences.

All works can have errors. Newer works, while not perfect, have a chance to catch errors in older works and improve.

In addition, none of the numismatists have seen all the coins of a type (except in the case of limited edition types). Later numismatists often have the advantage of collecting information from multiple old source and new images and are able to have a better understanding of what was produced.

I'm not sure if it fits your definition of an original historical source, but look at Brandenburg-Bayreuth KM# 247. Research at the time created that type definition, but the coins covered by KM# 247 are really four separate distinct types.