The only part of a mechanical seal that is supposed to be sacrificial is the carbon face. The seal should run leak free until the carbon face is worn away. If the seal leaks for any other reason, we consider it a premature failure, and can be corrected.
Two hard faces are selected when carbon is not acceptable in the application and you have run out of options. You are then trying to get the longest life you can.
The only variable in a seal life should be the lubricating quality of the product you are sealing. Hot water, many gasses and most solvents are typical non-lubricants.
With all of that said, the fact is that in excess of 85% of mechanical seals fail prematurely. When seals are removed from the running pump, most of the carbon face is still intact. Little face wear is the rule not exception.