All LEDs are photodiodes too, certain degredations of parts or poor circuit design could lead to the display turning into a switch.

Eh, I don't agree.

LEDs are diodes (Light emitting diode). Certainly this was a cost saving measure, but it's not a bad assumption that the LED wouldn't allow reverse current flow.

Don't underestimate the appeal of saving one cent per unit. So long as the costs are externalised, anyway...

It’s not exactly designed to fail, they just don’t care. If they could add a one-cent part that made it fail sooner, they wouldn’t do that either.

Capitalist profit motive strikes again. The invisible hand expands tech and the visible hand keeps making tech worse.

People usually respond to this by saying that it would be absurd to suggest the company did this for its own benefit, when anyone who engineers knows these are often caused by revising design to minimize costs... and increase profits.

In almost every system with failsafes there will be conditions that can bypass them. The goal is not to make it impossible for the unsafe condition to happen, but to make it so that in the expected uses the failure will not happen.

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.

In that situation one of the switches should short the mains voltage and blow the fuse when the door is opened.