I wouldn't be so harsh about the comparisons, because we are after all all humans and whether we like it or not, we do lot of things awfully in a similar or the same manner even though there is much space in the mouth...
One problem to compare it with the English "ing" ending is that it will provide us only with one nasal vowel, as you say it, but I'm personally quite sure that if English provided us also with other vowels in this manner, they would sound very similarly to the French nasals. I think that the thing where the French nasals are so distinct (personally) is that they are so long and therefore "extremely" nasal, because the tongue has enough time to get to the ideal position - it may not have so much time (hard to say) with the English nasal "n" and probably that would be the same in Latin. Less extreme French nasals
But that is my hypothesis (which is, I think, harmless and not close to being unacceptable
).
But I know that any explanation based only on theories and less on data may be considered as a simple one, less credible. On the other hand I think that the space to maneuver with the nasals in the mouth is not that big that we should feel forever desperate as never being able to achieve something that may be as close as possible
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Anyway, an interesting topic! And surely we could link this thread later when the discussion arises
"why we are so bad boys (and girls) as not pronouncing the restituted pronunciation with half-cutted m/n's"