If the truck had exploded, that would be way too much in line with the overused film clich of exploding vehicles.
Not
exploding is a powerful statement by the director not to fall for such hackneyed stereotypes.

Spielberg said that he wanted the truck to roar down the cliff like a monster a la Godzilla. We can see the truck sort of howl in agony and it "dies." Fantastic film.

The tanker would have had to beBut empty for the truck to handle as it did,and go so fast.But the fuel tanks for the engine should have ruptured and burned.

Truck was wmpty the whole time. It was someone pointed out in the beginning with the scene of Mann first passing it. It gave a hollow sound of it not carrying any load.

Don't forget this was a made for TV movie, back when 99.99% of them were absolute cr@p. It was both shocking and exhilarating to see something this good on TV back then.

You consider it a masterpiece ? Really so ahead of jaws , raiders of lost ark and Schindler s list ? That's crazy . Those 3 films of SS are masterpiece and duel to me is just an ok picture .

It wouldn't have been as effective. Like an orgasm, it would have been incredible but very short lived. Once the explosion happened, pretty soon it would have been just burning fire and nothing else.
To see it go down the cliff in slow mo in detail and then to see the aftermath - the gravel hitting the camera, the tyre slowly turning until it stops, the oil (or was it blood?) dripping from the steering wheel, the fan in the cabin still operating as per-normal when everything else was dead. And after that you see Mann surveying at the clear wreckage and breaking down.
Also you do ask whether the truck driver survived even though you know he couldn't have. An explosion and fire probably rules that out completely.