In terms of cost, given that the cost of running the railways is underwritten quite substantially by the tax payer, I am reasonably most tax payers would wish their money spent wisely on the most effective way to move traffic by rail. If the railways don't continually modernise they become (or remain) vulnerable when governments tighten the purse strings.But my point is, why shouldn’t some parts of it be a glorified museum? Providing it isn’t imposing constraints on what can run, what’s the problem? As regards cost, it’s no different to listed buildings costing more to manage and maintain - the extra cost in considered worthwhile.
It isn’t so much the semaphores themselves, but the whole way of working. In the case of Shrewsbury in particular there seems to be little appetite to try and design a modern scheme.
I wouldn’t advocate keeping old methods of working on large amounts of the network, but in a few particular locations it’s hardly a major hindrance. Cornwall would have been my first choice, perhaps the S&C another. Grantham to Skegness perhaps.
If it isn't the semaphores themselves but the way of working, this goes way beyond the scope of Listing. Buildings are listed because of their historical and/or architectural significance. Listing also comes with quite significant restrictions on alterations to those buildings in order to protect their fabric and character. As an aside, I am a Local Authority Planner and have been for over 20 years. What you are seeking to do is protect things that are not immovable objects but more an atmosphere. Even Shrewsbury doesn't have this any more. The gantry at the northern end of the station has a mix of semaphore and colour lights, for example. Even historic station buildings have moved with the times and been refurbished to meet the needs of a 21st century passenger rather than a pre-WW2 one.