I would be surprised if the footbridge was being modified, as noted it is in poor condition, but as nothing seems to be happening on it then that may well be the case. I think you may be right with the age estimate.
I would have thought if it was being replaced, it would be replaced pre-wiring and definitely pre-energisation. If it was to be replaced with an accessible footbridge I would have also thought it would have been started by now
I believe early in the East Kilbride electrification scheme plan, Giffnock and Clarkston bridges were planned to be replaced with accessible new ones. Clarkston is of the same vintage as well. Don't know if this is still in the scope or not.