Much/Many Fewer?

I made another haphazard survey of standard authors. I don't know how to respond to the charge that English has changed since these authors lived; perhaps someone can show me how to choose a living author and then how to search his works. I also see no indication that the original post contemplates a recent change in syntax. Anyhow, here's what I found, searching for uses of many (or much) followed by more, fewer or less.

(1) “Much more” is common, but always meaning “far more,” not “several more.”

From Pride and Prejudice:

• You have shown him off now much more than he did himself.
• It would surely be much more rational...
• I can much more easily believe

From David Copperfield:

• very much more common in books
• made his angry face so much more angry
• they would have said much more about her

(2) “Many more” seems to suggest “several more”

From Tom Jones, by Henry Fielding

• I warrant he hath many more bastards to answer for
• and though his face bore some marks of Susan’s fist, and many more of her nails, he rather chose to be contented
• a woman who hath not only all the good qualities I have mentioned, but many more.

(3) I did not find any instances of "many fewer" or "much fewer."

(4) "Much less" is sometimes used, but to mean "far less," not "several less (or fewer)"

• much less would he have excluded all other objects and ideas from her mind... - The Scarlet Letter
• Mr. Wickham has deserved much less attention from Mr. Darcy than he has received. – Pride and Prejudice
• changed into a much less agreeable aspect – Tom Jones

So I conclude that the advice quoted in the original post is not correct. The distinction between many and much is not that much can modify an adjective while many cannot. The distinction is in the meaning of the expressions, and perhaps a stylistic avoidance of the puzzling "many (or much) fewer."

The quoted explanation is wrong for a few reasons

Most obviously, it disregards current educated usage. So it's descriptively inaccurate. Ignoring that, there is also a problem with the logic of the argument. It rests on an assumption that many cannot be used adverbially, but that seems to be belied by the existence of phrases with "many more", like "many more apples".

In current usage, "many fewer" is more common than "much fewer"

Both much fewer and many fewer are used today in formal written English; apparently much fewer was originally the only form used, but many fewer is more common nowadays.

Consider the examples in (1a-b), which concern many and much as intensifiers of the quntifier fewer.

(1) a. Nowadays our newspapers carry many / much fewer ads.
b. They had many / much fewer (of these features).

The rivalry between the two is the result of a long drawn out replacement process leading from the exclusive use of much in earlier centuries to the predominant use of many in formal written English today.² This prompts the following question: are there any environments delaying or speeding up the ongoing change? Informal observation had suggested to me that much fewer occured more freely in cases like (1b), where — unlike the determinative usage in (1a) — there is no nominal head following fewer. This hunch was confirmed within minutes by the Google search summarized in table 1. [...]

2 A more detailed diachronic and synchronic analysis of this phenomenon is under preparation.

("Determinants of grammatical variation in English and the formation / confirmation of hypotheses by means of internet data", by Günter Rohdenburg, in Corpus Linguistics and the Web, 2007, edited by Marianne Hundt, Nadja Nesselhauf and Carolin Biewer)

The further analysis seems to be in “New Departures”, by Günter Rohdenburg and Julia Schlüter, in One Language Two Grammars, 2009, edited by Günter Rohdenburg and Julia Schlüter.

Rohdenburg and Schlüter mention that "far fewer" is another option, but choose not to compare it with the others. Using databases, they identify two main factors that seem to contribute to the use of "much fewer": it's more common in American English than in British English, and it's more common in both American and British English when there is no following noun phrase. Even in the conditions that are most favorable for "much fewer" (American English, no following noun phrase) it is less common than "many fewer", with only 6 occurences of "much fewer" in these circumstances compared to 16 occurences of "many fewer" (Figure 19.7, p. 372).

As evidence for the historical precedence of "much fewer", Rohdenburg and Schlüter include a citation from Dwight L. Bolinger's Aspects of Language (1968: 127); they say he "writes that 'many fewer is next to impossible' " (Rohdenburg and Schlüter 373).

Other facts that I think are relevant:

"Many" might be an adverb after all. It seems to be used as such in the clearly established phrase "many more"

While phrases like "much more people" do seem to have some use, "many more people" is much more common. The Oxford English Dictionary (OED) has a couple of examples of many followed by mo/ma (an older form of "more") from Middle English, and an example of the exact phrase "many more" from 1559 (W. Cuningham Cosmogr. Glasse). So, "many more" is clearly established in both modern and historical use. The OED suggests that the construction many more "probably arose as an appositive collocation of adjectival many with more (Middle English mo), e.g. ‘there were many, more than..’." This might explain why "many fewer" didn't come to be used until later on: the expression would contradict itself if many is read as an appositive adjective. But this explanation is only historical; the OED does classify "many" as an adverb in the modern expression "many more".

"A little more" and "a few more" are distinguished; "few more" may exist

I tried looking at the opposite case with "more". We do say "a little more [non-count noun]" and "a few more [plural count noun]" respectively. But the indefinite article "a" may make a difference. "Little more [non-count noun]" without an article does exist, but it's not common. I have found an example of "few more [plural count noun]":

you will discover that he has few more ideas than a brute

(The Excellence of Christian Morality by William Bennet, "Appendix" p. 24)

The quantifier "fewer" could be classified as a determiner rather than as an adjective, but I don't think it makes a difference to the argument

Edwin Ashworth left a comment saying

Few/er in 'few/er' apples wouldn't be classed as an adjective nowadays anyway, so the analysis falls down.

However, I don't think it makes any difference, since as far as I know determiners may require adverbial modifiers just as much as adjectives do. Wikipedia mentions that possessive determiners like "my" etc. can be modified by the adverbs "more", "less" or "(as) much".