On the contrary, it would increase fatal accidents. Airplanes require airflow over the wings in order to fly. Once that airflow gets too slow, it detaches from the wing and becomes turbulent. This is an effect called aerodynamic stall. When that happens, the wing loses lift and can no longer hold up the airplane.
From high altitudes, in most airplanes it is normally very easy to recover from a stall and all pilots practice doing this. Simply pitching the nose of the aircraft downward will trade some of the aircraft's altitude for airspeed and the stall condition will be recovered. However, if you're flying low, you don't have as much extra altitude to spare and you're more likely to crash into the ground before you can recover from the stall. So, by flying low and slow, you have much less airspeed margin to work with before stalling and also much less time to recover from a stall should one occur.
In addition to the wings being less effective at low airspeeds, the aircraft's control surfaces are also less effective (and, if you fly too slowly, those can stall, too.) The slower you fly, the less control you have in all 3 rotational axes. This further reduces flight safety by reducing the pilot's ability to maneuver the airplane.
Thus, far from decreasing fatal accidents, flying low and slowly is actually one of the most dangerous things you can do in an airplane.
As far as the question about "going down" from 150 mph at 5,000 feet vs. 500 mph at 30,000 feet, in the best case, this will have no effect on survivability. In most cases, it will make survivability much worse. In the case of the aircraft becoming completely uncontrollable without any possibility of recovering controllability, the situations will be the same, as everyone will die in both cases. There is little-to-no survivability in either case. However, the aircraft suddenly losing all controllability while cruising is (thankfully!) an extremely rare failure mode for airplanes.
Most incidents that happen during cruise (which are already a relatively small minority of aircraft accidents) are aided by having more speed and altitude. As discussed above, the speed keeps the airplane flying and makes it more controllable by the pilots. Altitude buys the pilot reaction time.
Consider, for example, the case that all of the airplane's engines lose power. If you're flying 150 mph at 5,000 feet, you have only a few (maybe 3-5) minutes before you'll be on the ground, one way or another. Since you're only flying at 150 mph, you're unlikely to be able to safely reach a runway much more than about 10 miles away. Now consider the same engine failure, except at 500 mph and 30,000 feet. You now have probably over 20 minutes before you'll be on the ground and you're moving 500 miles per hour. Any runway within about 100 miles is now potentially an option for a safe landing and you also have a lot more time to try to get the engines running again before reaching the ground as well as to alert ground staff to have emergency equipment waiting for your arrival. The latter scenario is much more likely to be survivable. This is especially true if the aircraft is currently flying over rough terrain where a crash landing would not be survivable.