All of the years and hours on the generators and? I would think that the manuf. has considered all of these issues wouldn't you? However, if the WB relay has a typical suppression diode integrated and someone installed a relay without then I could see the issue being real. The Only device on that diode is one small relay.It wouldn't take a dead short one one side of the diode to fry it, just more current than it's rated for. If it's a 4 Amp diode and you're drawing 4 Amps though it, you're running on borrowed time. If half of the current flowing through the fuse was going through that diode, then the diode would fail around the same rate as the fuse would.
The other thing that can fry a diode is "back EMF" from something like a relay coil. When the current flow to a relay coil is interrupted, the magnetic field around the coil collapses. When this happens a spike of voltage is sent out both ends of the relay coil. It's like a micro-sized ignition coil and can cause a spike of hundreds or even perhaps a thousand volts. If the diode is not rated to handle this spike, then it will fail when the spike arrives at the diode's leads. A small capacitor to ground can eliminate this spike before it becomes a problem.