In actual practice, fewer has always been restricted to count nouns, but less is often used with count nouns, too, especially in certain constructions.

So no native speakers would ever produce your first sentence, but a great many would produce your second, and they have been producing such sentences for over a thousand years.

The use of less with count nouns is indeed a part of idiomatic English—natural, idiomatic language is seldom as neat and tidy as prescriptive rules. But the rule is indeed something we can pin on some eighteenth-century dude. The history is pretty clear on that point. See this entry in Merriam-Webster’s Dictionary of English Usage.

Unfortunately, I’m not sure what you mean by “fluid” and “complex”, but the biggest reason why less is okay with things like units of time and measurement is that they refer not to discrete, countable things but to amounts of stuff. Your argument is attempting to fit the facts to the rule rather than the other way around.

Usage came first, and the rule came along much later to try (mostly unsuccessfully) to force the use of the two words into complementary distribution on the usage. People “break” the rule not because it’s simpler, but because the rule runs counter to the natural idiom of English.

How about “She has less bananas than I have”?

This doesn’t seem to be something we can pin on some 18th century dude, rather, it involves grammar at a deep level and is fundamental to idiomatic English.

So. How do we explain “10 items or less”? Probably because a statement like, “Sam has less than ten days to live” is more fluid than “fewer than ten days left to live,” which is a more complex construction.

I think it’s arguable that in most cases a native speaker will employ “less” and “fewer” in the correct way, and break the rule mostly in cases where the phrasing is simpler.

I like this comment you made: “People will find any excuse they can to discriminate against others, and language provides an easy way to distinguish those in the know from everyone else.” It sums up nicely another peeve I have lately about strident “grammar nazis,” which is the way in which they tend to divide the world into “stupid people who don’t know grammar” versus “smart people who do.” It’s just not that simple, I think.

I agree that they’re needless, but that doesn’t necessarily mean that they don’t provide any information; they simply provide information that’s already provided elsewhere. This sort of redundancy is harmless and may possibly be helpful if it helps a reader or listener anticipate what you’re going to say next. But any benefit is probably measured in milliseconds, so I’d say it’s a waste of time and effort to invent rules and then try to get everyone to follow them.

Strictly speaking, we don’t need rules like this, but I think that, to some extent, they’re inevitable. People will find any excuse they can to discriminate against others, and language provides an easy way to distinguish those in the know from everyone else. I think the only thing that can be done about it is to change the way we teach language in school, but that’s much easier said than done.

On a related topic, the less/fewer distinction, along with who/whom and other similar rules, strike me as needless — they don’t provide any information. They’re strictly niceties. In my mind, they will change soon rather than later, and English will be the better for it.

I’d like to hear your opinion (or a link) on this. Do we need grammar rules that serve little purpose other than giving people a reason to criticize us?