Interesting points, thank you @audphile1
A Etherregen reclocks the signal, but a normal switch has not necessarily a better clock than the router. Also, electrical noise WILL pass the switch. But not an Etherregen.
Conductivity? Yes, if cable is so lossy that frequent retransmit have to be done, I agree, otherwise litterlally yes/no/binary/works/works not.
Neglible for a word document? I beg you pardon, I demand and expect a 100% copy and not a 99.99999% ....
Jitter? Yes, I agree but cant see how a cat6 vs Cat8 would change that. Both Cat6 and Cat8, at the short distances typical in a house, deliver a bit‑perfect, full‑duplex link with far lower error rates and latency variation than anything else in the path. Only if an extreme noisy envoironment triggers retransmit, jitter might be afftected.
I appreciate your thoughts, but can see how your points explain anything. Again, I am not (NOT) stating that there cant be any advanage, but it would be outside of what you explained.
A optical to cat6 converter can change jitter however, I agree.

