20 year old wire

[Brion Toss]

Hello,
First, the splicing is unlikely to be a limiting factor, unless it is served such that water can be trapped long-term inside. Next, the scantlings do sound absurdly huge, so as long as the tune has been good, fatigue is not likely to be a big issue. But of course rigs are often very badly tuned. Next, what is the wire's construction? 1/4" might be right for at least some of the wires in the rig, if the wire is 1x19. But if you happen to have 5/16" 7x7 or, worse yet, 7x19, you have a wire that is not all that much stronger than 1/4" 1x19, and is a lot more corrosion-vulnerable (more elastic, too). Next, we also know nothing about the quality of the current wire. Is it type 316? Who made it? What was the average break strength, as opposed to the rated strength?
So many variables. The short form is that it is probably time to replace it, regardless. 20 years in warm climates is a lot, whether it got heavily used or not. And where you are will only accelerate things. Before replacing anything, though, run the numbers. It is vanishingly unlikely that every wire in that gang should be 1/4".
Fair leads,
Brion Toss