Simply slowing the cars down is not the answer to how to make them safer. The incident that prompted the switch to 1000' is a perfect example. Scott Kalitta would likely still be dead if he had been going 10, 20 or 30 miles an hour slower. Running into concrete at 250 will kill you just as well as 280. The key is to not run into the concrete.
Racers die in crashes because of blunt force trauma, penetrating trauma, fire related injuries or some combination of the three. Slowing the cars only partially addresses these issues.
Fire related deaths (i.e. burns, inhalation of toxic gases or lack of oxygen) are almost non-existent.
Penetrating trauma such as getting stabbed with a chassis tube or the support structure of a funny car body can happen at relatively low speeds. A look at Mark Niver's crash in Seattle illustrates it doesn't take 300 mph speeds to generate enough energy to turn the car itself into the weapon. Just to be clear, I don't know that Mark's cause of death was penetrating trauma but it doesn't take a big stretch to imagine part of that chassis spearing the driver. Bob Tasca's efforts to build a funny car body without the metallic support structure was in response to this issue.
Blunt force trauma is probably the most common cause of death. This can be the result of running into an object outside the car (Scott Kalitta), getting hit by something (Darrell Russell) or extraordinary internal forces (Eric Medlen). It's difficult to find accurate records but I don't believe any of these drivers exceeded 310 mph on the runs that killed them.
Anecdotal evidence aside I think we can all agree that there is no "safe" speed. NHRA and the competitors are better served by creating systems and an environment that limits as much as possible the forces on the driver and the possibility that an object can strike the driver. Canopies, the Electrimotion safety system, driver compartment padding, decreased severity of engine/tire damage and improved shutdown/containment systems have done way more for driver safety than reducing speeds by 20 or 30 mph.