Just thinking out loud, could we assume that we won't see this sort of damage under similar loads and road conditions with the new Navara seeing as though it has rear coil springs?
They won't make any difference. The problem is what is on the end of the chassis back past the axle. When the chassis falls and is eventually brought to a stop by the springs, bump rubbers or aftermarket air bags, whatever is behind the axle does not want to stop. It thumps the end of the chassis down hard if the material on it is heavy enough. The chassis then tries to pivot on the axle and tries to lift the other end.
It is the same if the front of the chassis drops down. It pivots on the axle housing and instantly tries to lift the rear end . If the forces generated are beyond the chassis's design limits, it wll bend. It is heavy material on the end of a lever that is causing all the problems. The lever is the distance from the axle back to the material.
Springs can't stop that. Moving the axle back to the far end of the chassis would because there would be nothing extending out the back unsupported. That is probably not all that practical though.
You could try a simple little test of this with an egg carton. Place two eggs in it at one end only. Hold the end of the carton with the eggs in it against your body in a horizontal position with your thumbs on top and a couple of fingers on each hand under each side of it. Rock your fingers slightly so the other end of the carton moves up and down. Note how it feels than turn the carton around and do the same again.
You will notice a considerable difference when the eggs are further away from your hands. The heavy end of the carton does not want to stop when it is going down and it is harder to lift it up quickly. That is what is happening to the end of your chassis. It has been designed to resist a certain amount but it will bend if it is excessive.
I think a lot of aftermarket suspension advertising has lead many owners to believe they can put anything on the back of their car and the suspension will fix it. It will lift the car and maybe improve the ride but it can not remove the excessive weight way down the back that is flexing the chassis up and down beyond the limits that the factory engineers designed it for.