The sad thing is that a local engineer had costed a tunnel instead of the cable-stayed bridge that they are currently building downstream, and it came out cheaper, and of course could be used regardless of weather. But the politicians had paid for a study which was based on the assumption that a bridge would be needed, and so it is. It is claimed to be usable in all weather conditions, but I expect that within a year that will be shown to be a lie. But the tunnel proposal will be conveniently forgotten.
Meanwhile the existing road bridge remains closed for a while yet because its designers appeared to be somewhat ignorant of metal fatigue. I can just imagine the chaos at the Kincardine bridge and the mediocre roads from there into Fife.
A tunnel would have been truly interesting. While the costing was for one of those that you build on shore and sink into a trench on the sea bed, bored tunnels are very much in fashion right now. A TBM would need rather hard teeth for boring through whinstone, but I think that is possible with carbide teeth. Might need impact action, possible with hydraulics, but not good for the workers, so it would probably have to be remotely controlled. But why not? It would just be a combination of existing proven technologies put together for the first time. If an innovator like Greathead (inventor of the tunnelling shield, forerunner of the TBM) was alive today, he would jump at the chance.
I confidently predict that when the road bridge fails completely, as it will, there will be such public outcry that its replacement will be a tunnel. (It is still needed in current plans, when the new bridge opens.) What will bring down the 1960s bridge will be the direct result of politics.