How big a threat is ESD, quantifiably?
I'm your age and am working (though only recently) in the field of electronics design. However I believe I can't give you what you call a quasiquantifiable experience rather than sharing some observations and reasoning.

As per my experience I, too, never witnessed electronics devices damaged by ESD. However...


this doesn't necessarily mean ESD protection is overrated. Fact is most electronics components and devices are built with ESD protection in mind. They can resist to ESD to a certain level, which means it could just be that the average level of ESD in daily life is well taken care of.
ESD causing some form of wearing out is conceivable, i.e. high voltages in repeated strikes might very well and progressively cause permanent damages in the long run -- similarly, flushing too much current through a diode destroys it, right? However one would have to analyse the silicon layers of decap'ed chips under a microscope. I have no idea whether that has been done.
Quantifying ESD is tedious because there are so many circumstances in which "things" rub against each other and build charges let alone how many materials, humidity conditions... The industry can only provide well-controlled testing environment that at least match observed and known live conditions.


As for your students, who seem to have yet to observe any damages made to their boards, caution tells that's no reason enough to suspect nothing happened -- there might be lurking damages, still, but they're not observed if they occurred to unused parts of their hardware or if their boards are slowly degrading, for instance. In this case only time will tell.

By the answer you linked to it is obvious humidity plays a huge role in reducing the accumulated charges, for instance. Common sense is to at least use ESD protection while carrying bare electronics devices. If damages don't seem to be observable caution rules, still.

As for the damages of ESD, I like this video demonstration of a youtuber called Kevin Darrah. He manages hard to destroy an ATmega328 by simply rubbing his feet against the carpet and touching a pin of the chip. But the chip ends up destroyed. It's interesting how damages seem to accumulate to a point the chip is definitely terminated.

EDIT: In another video, he also manages to destroy N-channel MOSFET transistors doing the same exercise. He also demonstrates a basic protection and tries it.

That, IMHO, is enough to treat ESD seriously. Maybe do some experiments with your students?