The original posting raises a number of questions:
How do you know the driver is incapacitated? Just because he has not communicated with you, does not mean he is incapacitated. As per a recent incident, he may be on the phone dealing with the issue. Likewise the guard (if there is one) may be protecting the train. Exiting the train may well make the time to rescue you or get you moving again considerably longer.
How do you know you are in imminent danger? If there is a fire, or smoke, then OK. But someone raised the issue of where a train is derailed across the other track. In such a case, by the time you have recovered your wits, have worked out how to release the doors, and have got out and to a position of safety well away from the train, it will likely be too late. If you haven't been hit by another train within that time, it is highly unlikely that you will be, as the signaller will have become aware that something is wrong and protected you.
Have you ever tried exiting a train from a passenger door when not at a platform? The cab doors have steps to help the crew get out, but even so in places where trains are not intended to stop it can be awkward. Passenger doors don't have steps, it is a long way down, and you are likely to be getting out onto a slopping ballast shoulder. You could very well end up with a sprained ankle if you land awkwardly. Even worse, if you realise that you have made a mistake, it is going to be even harder to get back on.
What about everybody else? If you try and open the external doors, someone else is bound to ask what on earth you think you are doing. At worst, you could start a panic. At best, by being "the person who knows what they are doing" you will probably by default find yourself in charge of evacuating a potentially large group of people, many of whom will be less agile than you, perhaps injured if the train has derailed, and some of whom will insist on bringing all their luggage with them.
What are you going to do after you have evacuated? It seems pointless to leave the train just to stand next to it! Even if there is a trackside access point nearby, it will be locked. You will inevitably end up treking off along the trackside. Leaving aside questions of which direction you are going to head-off in, this is where it starts getting risky if you are not experienced in walking trackside. There are no end of tripping hazards, such as loose troughing lids. At worst, you could find yourself in the path of a rescue train.
If you are an experienced railwayman used to working trackside, then you will know that it is safer to remain on the train. If you are not experienced in walking trackside, then it is almost certainly safer to remain on the train.
As others have said, and as per the official guidance, you are far, far better off moving to another coach if you think that you are in danger in the coach you are currently in. Exiting the train should only be the very last resort. In almost all situations, you are far better off staying on the train.