You miss the point. So all the drivers refuse to work the trains and new legislation comes in saying they must. They all still refuse. FGW sacks them all using this new legislation. They now have no drivers and start recruiting more meaning FGW have no train drivers for at least 18 months. Never going to happen is it. Let's be realistic here.
I fully accept that it's very unlikely to happen but, with respect, I certainly haven't 'missed the point'.
If new legislation made certain areas of public transport into 'essential services' where strikes, or similar disruptive action, was outlawed then the train drivers in dispute would be up against the law, not FGW. I doubt that many drivers would want to risk a criminal record, so would (albeit very reluctantly) take the trains out after all.
Although I've agreed that it's very unlikely to happen, it is still a possibility if the dispute remained in stalemate - presumably, you are just assuming that FGW will back down (and leave the issue for the next franchise holder) ?.