It's widely known there are often problems around or not far inside the top of the swage of the bottom terminals, in salt water
use in the tropics - concentration cells get set up by the water
continually evaporating away leaving the salt
People try to keep water out of the swage by various means but it's problematic partly as has already been mentioned because it tends to set up oxygen starvation leading to crevice corrosion
- ideally a boat used in these parts
of the world, I think, would be best with discontinuous rigging and replacing the lowest elements perhaps twice as frequently as the remainder.
Of course the entire forestay cannot generally be rendered discontinous, so the whole stay qualifies as a 'lowest element' and in any case any stay on which sails
are flown should be replaced in toto
more often, given the fatigue-inducing fluctuating loads to which they are subject.
In the bridle-rigged backstay case, you could try and get by with not replacing the upper panel as frequently, but personally I think the junction is probably too close to the salt
source, ie the sea surface....