Can you provide some details about the pump failures?
Are you running along, everything fine and then the pump suddenly stops working while you still have plenty of gas in the tank?
Do the pumps fail to work properly at the start of the season after having sat all winter?
Do you try to stretch the tank a bit too far, run out of gas and then can't get the pump to prime again?
I just can't believe that the level of gas in the tank will have any impact on the life of the pump. The work the pump does is a function of the difference between the pressure at the input and the pressure at the output. The effect of the gas level in the tank will be approximately 1/3 PSI per foot, so if your tank is three feet deep the pump will have to create one additional PSI to maintain the output pressure. I suspect that that is way less than 5% of the total pressure the pump provides.
What is common on fuel pumps, especially in a boat, is for them to fail to prime. Fuel pumps are designed to pump liquid, and they don't pump air very well. When you run the tank dry the pump will have to pump air until it can draw gas from the tank. A boat will have the gas feed draw from a dip tube on the top of the tank, and then there is typically a fuel filter/water separator between the tank and the pump. A slightly worn fuel pump might not be able to create enough suction while pumping air to draw the fuel.