“For info for non pilots reading the 738 can be stopped in approx 3000ft or 1000m after touchdown with flap 40 and autobrake max on a dry runway.”
Plus some small print.
Providing the aircraft crosses the threshold at Vref at 50 ft, touches down within 1000 ft (HUD / Autoland distances are longer), at the assumed touchdown speed (Vref -7?), and that the spoilers / airbrake / reversers are deployed without delay (1sec buffer) and maximum braking used until stopped. And that the tyres and brakes are in reasonable condition, the runway surface and friction characteristics are as assumed by the performance chart, that the wind is as reported, and no doubt a few other minor items such as aircraft weight and all systems remain serviceable.
In practice, because the aircraft and flying techniques do not meet the accuracies above, an aircraft operation is planned to stop within a certification distance consisting of a ‘theoretical’ minimum (similar to the above) plus a factored safety margin. However, even this distance is often exceeded in overrun accidents due to a combination of factors, dominated by false belief, assumption, inappropriate action, or unknown / unreliable values.
“… long runway ... plane off the end. How ? Why ?” Not even a HUD will answer those questions.