It isn't the current which kills starter relays, it is the back current which arcs contacts to an early grave. The current flowing through a solenoid coil creates a magnetic field. When that current is interrupted, the collapse in the magnetic field causes a reverse current, much stronger than the original. The original starter solenoid has a small diode which is supposed to stop this back current. But age and repeated use can cause it to fail open, allowing the back current to arc weld the contacts. This failed diode has no symptoms. Caveat detonator. Watch out for things that go boom.