1990 Chevy Cavalier Cooling system

1990 Chevy Cavalier 4 cyl Two Wheel Drive Automatic 196K miles

I replaced water pump at 148K, it started leaking at 196K and when I removed it the impeller vanes were worn completely off! What could have caused this? Also if this car gets 1 gal. Low on coolant the engine sputters and dies at stops when it's cold, add coolant and it runs fine, is this a designed in safety feature and how on earth would it work? Curious, Thank you