"What desired watch(es) will you never buy the genuine of, and instead will opt to keep, buy or modify the replica counterpart?"
Any modern (sapphire) rolex.
I like the 16610 but would never buy one unless it was a real bargain.
The only 16610 replicas I have are F520117 Noobs and a few assorted not noob 21j and swisseta models from 12 or 15 years back. Do not wear them, like vintage better.
Do not care for the fat lug ceramic models.
"To ask the question in another way, what rep watch do you own, that satisfies you enough such that the gen one becomes more or less (though maybe never completely) redundant?"
MBK Frankenstein 5512 and 5513. Both have 15xx movements/ST bez kit/GS crystal/ST case tube/genuine crowns. Imho this makes them just about as good as a high priced genuine watch without the worry of putting a scratch in the case or corrosion creeping up on it.
The only worthwhile improvement would probably be to jerk the 15xx movements out and stick freshened 2846 Etas in them...