The old Severn bridge was retained and refurbished after the new one opened, they should do the same for the Forth. For such a crucial route, that kind of redundancy is worth paying for. The Forth road bridge is about the same age as the old Severn bridge and probably a similar design, so this proves your view that refurbishment is possible.
I think the Severn, Humber and Forth bridges all are of similar design: all three have the same issues with individual wires snapping and AFAIK have all had similar acoustic monitoring and dehumidification systems installed.
The Forth bridge cables have 11,618 wires wrapped into bundles, and they have apparently found 93 broken wires since 2006 (may be out-of-date stats now)
However, the current issue is apparently due to structural cracking in the bridge deck support truss itself (which does look significantly different from the Severn Bridge deck) - the bridge apparently was designed with a max capacity of 30k vehicles per day, which is regularly exceeded and could raise to almost double it's design capacity
Earlier article on cable issue [www.scotsman.com
New Civil Engineer article here: [www.nce.co.uk
Edited 6 times. Last edit at 17.04hrs Sun 6 Dec 15 by geoffers.