Wire Resistance Degradation or is it Really Insulation Leakage

Well, both can be true. It depends rather on timescale.

(For # 1 above )Is there any study, graph or trend we can use to estimate age and degradation with or without some parameters may be, temperature, environment perhaps.

I only see classes of insulation vs lifespan vs operating temperature

That graph may well be applicable.

1,000,000 hours is just over 114 years, btw., which is beyond any individual's working lifespan and there aren't too many electrical installations that old.

Both are true, they both happen. What baffles me is why you think one must be true thus making the other a myth. Both open and short circuits are failure modes in electric circuits.

If you're close to the ocean, the salt in the air can be a problem with wire insulation.

Sure thing, but I am most interested in the wire core. How it deteriorates with time of use and from the two what is design factor to due or give up first. Is it the insulation, or the wire properties.

The number one degrading factor on insulation is heat, and it is accumulative. Periodic overheating of the insulation will eventually cause failure. Back in the days of high quality over high profit, copper communication wires were coated with silver or tin.

Silver oxide is a very good conductor, and even if it turns black, it is still a good conductor and protects the underlying conductor from corrosion.

Insulation is constantly improving in durability and are mostly application specific, such as overhead, underground, underwater, ect.

Some wires are designed to flex, and are made from many very fine conductors in parallel. There is no one size or rule that fits all.

The wire itself I don't think degrades intrinsically over time if it is operated within spec, however there are several environmental factors that could degrade the wire most notably at the connections and insulation which are subject to oxidation and uv rays, not to mention vermin attacks such as ants, mice and rats and other insulation curious beasties...movement through manipulation and bending, vibration etc, can weaken the wire as well...So if the wire is protected from the environment by an intact coating and is not moved a lot, has a corrosion free connection, it should remain viable indefinitely...

..."Underground cable contains insulation material which has a design life of 30 to 40 years. Overhead lines are designed to provide safe insulation using glass, porcelain and physical air space. These materials have a design life of 80 to 100 years."...

Link...

Wire resistance can increase depending on the impurities in the coil wire whether it is copper or aluminum. Insulation can deteriorate and effect the winding as well as the insulation break down.

On a separate note we had black insulated cable where the moisture and the carbon in the colour caused the copper cores to corrode away to the point where the current carrying capacity decreased markedly.