> Jaques is the French spelling of Jack/Jaak/Jak/Jaq. They're all pronounced the same
They're not, though. The French pronunciation of 'j', as in the word Jaques is /ʒ/. In English, 'j' at the beginning of the word 'Jack' is pronounced /dʒ/. And 'Jaak' makes me think of Dutch, where that 'j' is pronounced as /j/.
In the real world, the descriptivist realizes an individual's pronunciation of the concept labelled Jac/Jack/Jacques/Jacq/Jak/etc. depends much more on the their personal context and stylistic choice than the spelling used.
I've heard many folks (American and otherwise) pronounce "Jack" many times in my life, and the range of utterances very comfortably includes Pépin's own "Jacques".
There’s no single way “native” speakers say any word, and the fact that you think there is shows you have had no exposure to the massive diversity of American accents.
What dialect of American English treats /ʒ/ and /dʒ/ as allophones at the beginning of a word (or any other context)? You've already (weirdly) accused me of being a prescriptivist, here's your chance to counter with some descriptive evidence of the kind of variation you are talking about.
I'd love to be wrong, because I'd learn a new thing. Please, educate me.
I'm also from the US and I've spent quite a lot of time listening to people speaking, specifically listening for how they realize phonemes. Your experience doesn't at all align with mine, either what I heard or what I've read about dialect variation in the US.
Also not the point; Jaques is more difficult to guess the correct pronunciation of than Jaq. That's the point.