For low speed accidents (speed of impact less than 36 km/h), the energy absorbing elements can take all the energy, leaving the rest of the structure unharmed. For even lower speed, you see on the Stadler picture those buffers; they are the first elements getting destroyed. But they are easy to replace (in fact, they are designed that they can be easily replaced). For something more serious, the cab is normally reinforced plastic, and they do have spare cabs around (again, they are designed to be replaceable).

In case of an accident, it will be the insurance, together with the owner to decide whether a vehicle will be repaired, or scrapped and replaced with a new build, or completely written off (that would most likely happen when the vehicle has passed half of its lifespan).