The experts tell us that Nylon has a shelf life, so that even an unused rope 30 years old will be significantly weaker than a new unused rope.
Of course, those experts are the people selling rope...
When manufacturers coat nylon rope to prevent water
absorption, they usually coat the yarns before spinning or braiding the rope so the coating is all over the inside parts
as well. So even if a line has worn on the outside somewhat, there's still coating on the inside which helps prevent the fibers chafing on each other and extends the life.
The coatings you can buy aftermarket are usually water-based polyurethanes, such as Maxijacket and Armourcoat. They do an admirable job of protecting and extending life. There is also a silicone coating for making things slicker.
I don't know what coating is used for NER's 3-strand, but that's not the only consideration: twist, lay, and quality control vary wildly across manufacturers, and in my experience NER 3-strand is far and away better than cheaper brands. Even at full WM retail, I'd buy New England
before I bought some off-brand at half the price