Original rigging should all be replaced. But most of us have a mixture of old and new and here's the way I look at that, for what it's worth and nothing more.
If a rig goes over the side, it was caused by a defect visible to the eye. Debatable? Maybe. But stainless wire "never" parts in the middle like a piece of string. It's the fittings, from chainplates to toggles to turnbuckles to swages and mast connections, many of which differ in design.
We can't x-ray or tech-test the fittings, but we can examine them very closely. One pro rigging inspection teaches you what to look for, and how virtually invisible cracks and corrosion can be. A skeptical eye and a magnifying glass go a long way to getting confidence that your rigging is not a threat.
My anecdotal knowledge includes a large number of obvious issues that were missed. Typically, lee shrouds go a little slack in heavy air. elieve it or not, I know of two boats who had the shroud toggle pins
fall out. They tacked, the mast went down (or was about to). Another issue is cracks in chainplates, especially the cold-bent plate that holds the forestay on many Ericsons. That's easy to inspect.
I'm sailor, not a professional rigger, so to sleep at night I have to think I am competent to judge the obvious, and keep an eye on the standing rigging. Much is common sense. One pro rigging inspection is very educational.
Here's typical stuff. I missed the crack on this stemball, because it was on the underside. A rigger knew where to look.
A surveyor found this hairline swage crack. It takes a magnifying glass. Such a crack means immediate replacement of the shroud. Swages are always to be suspect.
Nobody could miss this exterior backstay chainplate. But everybody did, because it's hard to inspect when the boat is in the yard, and takes a dinghy when the boat is in the water.
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Aluminum castings tend to rot after 30 years in salt air. Many goosenecks are aluminum. Inspection is easy.
Any weld in stainless can be a weak point. Rust is a giveaway. But a little rust on Stainless 504, which Ericson used in the '80s, is often harmless and should wipe away with toilet bowl cleaner or similar.
Fittings screwed to the spars routinely come loose over time. Offshore, with the vang under constant pressure, several of these big screws backed off. It's a simple matter to tighten them, perhaps with a dab of screw glue. But you have to look. Same with the roller furling drum and the lower luff sections. Screws back out over time, and need to be tightened. Routine inspection and tightening can head off very expensive repairs.
Can we be aware enough to head off catastrophic rig failure? I think so.