Ok...here's what REALLY happens.
As I stated above, try not to run often below 1/4 tank,,,because..1)..the fuel keeps the electric pump cool.
We (at the shop) replace a dozen pumps per year (mostly Chevy) and many are due to the customer running it at about a 1/4 tank or less.
Also..2) EVERY fuel tank has 'some' crap in it, even at the gas station. That is why you should NOT get gas at a station that just received its load. The new load stirs up the crap that was on the bottom, so you may get some in your tank.
Also...yes, the pump picks up from the bottom. So...if you run around close to empty all the time, you are more likely to pick up the more CONCENTRATED crap in the tank.
The more gas you keep in the tank, the more diluted the fuel /crap mixture is, so the chances of sucking crap ONTO the strainer (pre-filter) and plugging it up are much less. As you drive and turn and hit bumps, the crap stays mixed thru-out the tank.
Think...one spoon full of Ovaltine 'lightly strirred' in a glass 1/4 full of milk with a straw. Now think full glass of milk, one spoon full, straw. get the picture.
And...I have seen fuel pump strainers that have gotten holes in them, and some that have come off, and just bounce around inside the tank. We figured this is what killed the pumps.