My experience is that electric grinders suffer much of their damage when the are *not* being used. (Unless you paid for it yourself, then they last for years)
Set on the floor, on the bench next to hot stuff, kicked, dragged, cord caught by passing object etc.
Do you have a place to put them?
Does your shop floor work flow allow for you to build something to hang your grinders on.
This might help both grinders and extension cords.