Stainless might not be the best wire to use: it can work-harden and will fail because of that (usually at the end fittings).
Galvanised wire is less prone to work-hardening, but is likely to rust sooner.
In either instance it's not a great environment for wire, particularly with the loadings, ie the weight of people exerting a force on them almost perpendicular (while bouncing along the wire).
If the wire is rigged too tight then the force on the anchors is multiplied massively. From memory, at 5 degrees off horizontal, a static 18 stone bloke would put a 3.5T force on the anchors (and the wire). I did the calculation way back, so take it as an approximation not gospel.
And if there is physical damage to a wire, that will significantly weaken it.
While 8mm galvanised should have a nominal breaking strain of perhaps 5T, with 1/4 of the strands broken (and likely the remaining strands weakened by corrosion), it's only a matter of time until a reasonable-sized person with a bit of a bounce is going to exceed the remaining breaking strain.
That said, my memory is that the wire was more than 8mm, but that was a while ago.
When re-rigged it should be with 12mm / 1/2", the largest diameter that a 'normal' Petzl pulley can take - that should give closer to 10T breaking strain and give a bit more leeway - but it should not be trusted once it starts to rust.
