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?
ESD is one of those issues where it is difficult to prove it really is the cause of failure on many occasions; rarely will a device be sent back to a manufacturer for an in-depth failure analysis.
That said, I have seen ESD failures, mainly in dry situations; as the breakdown voltage of air reduces with humidity, a given strike will have a lower chance of damaging a device, and this is the reason that 40% relative humidity is recommended as a mitigating step for manufacturing / rework to minimise ESD issues.
The biggest issue is that ESD can cause latent failures due to a part being damaged during manufacture, but not badly enough to fail testing. There can be, however, enough damage to cause early life failure.
I have seen many returned units where the only logical conclusion was ESD damage at some point.
Another issue is feature sizes; when I started doing electronics for money (1970) the feature sizes could withstand most ESD events; we had no special lab equipment at the time as I recall.
With current feature sizes at least 100 times smaller than the early 90s, the possibility of ESD damage increases dramatically.
Even ESD protection diodes are not really very strong; a few mA is usually enough to destroy them but the failure is often an open circuit and undetectable; the device is now at a higher risk of damage.
In short, ESD can and does cause device failure, but with all the variables involved we can only take precautions based on the possibility of device failure.
That is an interesting question...
Some grad student may have done some statistical analysis to determine the actual failure rates of devices due to static discharge, though I doubt those numbers are accurate. Analysing why a device failed is seldom done except in circumstances where a particular device fails often enough to make it warranted the study, either by the assembler, or the devices manufacturer. Also, in reality, a high number of device failures are chalked up to ESD, simply because no other reason can be found. If you have studied component life, you will know about the bath-tub curve. It is pretty much impossible to tell when a device just wont work whether it was ESD or just a weak part. You would need to open it up and look at it under a microscope for ESD damage. Even if you saw some, you could never be sure when or where it was actually damaged.
However, that is not the point. It's about cost.
We know that ESD can and will kill semiconductors.
the quantity of hardware that gets thrown in the plant's trash bin due to ESD
In reality, very few boards are trashed during assembly. Instead, attempts are made to debug and repair them. This adds a significant cost to the entire assembly process.
Further parts can be damaged but not enough to fail till after they are shipped. Field replacement is extremely costly, both in dollars, and in reputation.
In order to minimize those costs it is prudent to do whatever we can up front to ensure that as few boards as possible end up in the debug section. That includes ensuring all parts are handled appropriately during the assembly process. That of course includes minimizing the possibility of damage due to ESD.
The investments required to properly handle ESD are quite large, however, they are generally a one time cost which, when aggregated over years of assembly, give a significant return on investment.
That ultimately means, it really does not matter how frequent the failures would have been, one is too many.