Iṭrād requires names to follow one another naturally. If the reader has to reconstruct the family sequence repeatedly or if irrelevant words interrupt the lineage, the elegance of the construction weakens.
“Zayd son of ʿAmr son of Khālid” supplies an intelligible chain. By contrast, “Zayd, Khālid, ʿAmr” merely lists names and does not itself establish a genealogical sequence.
Even repeated use of “son of” is not enough. The rhetorical criterion is proportion, order, and fluent composition, not the longest possible list.