You could certainly argue that some sort of fire was inevitable, and that the fire would have been much less damaging if people hadn’t built their houses in such bad places.
But the law doesn’t really care about that. You can’t avoid liability by arguing what would have happened or what should have happened. If your negligence causes a fire and that fire destroys a house, you’re liable for it regardless.
California has contribitory negligence. If the court determined it was negligent to build or keep a house in these places and that contributed to the loss, it must determine the share of loss attributable to each party and reduce the award. In some states, a party must have be less than half at fault to receive compensation, but California doesn't have a minimum, if you are 99% at fault and the other party is 1%, you can get 1% of your loss compensated.
You could certainly argue that some sort of fire was inevitable, and that the fire would have been much less damaging if people hadn’t built their houses in such bad places.
But the law doesn’t really care about that. You can’t avoid liability by arguing what would have happened or what should have happened. If your negligence causes a fire and that fire destroys a house, you’re liable for it regardless.