But rarely do those failsafes protect reliably against 'the mainboard was splashed with salt water'.
Even with triple redundant relays, how do you know the salt water didn't just wet them all?
In this case it's a domestic microwave and the mainboard is housed inside the electronics enclosure, so covering the whole mainboard in salt water is not an expected occurrence in a domestic kitchen.
A domestic microwave is for use only on land, indoors, in a domestic kitchen, and in an unmodified form. In these conditions there is no conceivable way that salt water could saturate the main board, or bypass all the interlocks in another way.
Yes there are ways that all the safety systems can be bypassed, but not while a reasonable person is using the device as intended.
Source? People take risk in their day to day life and should expect to take risk. Why would they expect their microwave to be completely free of risk?
The design typically includes a mix of normally open and normally closed switches. If everything failed in the same direction (closed) it wouldn't satisfy the failsafe.
If you're spilling conductive liquid on the board, it's going to blow fuses anyway. It's more likely to short to ground than to short only to the precise path needed to activate.