K, but in fairness, that was Max Weinreich talking about Yiddish, which actually has multiple accepted forms (like Eastern Yiddish). Old English, for example, doesn't even use Roman letters for a while, and was heavily influenced by Old Norse (hence the Runic alphabet). Dialect vs. Language is an age old debate, but we're not talking about Creole vs. French or Roman Latin vs. Church Latin; there
is an enormous rift between even Tudor/Elizabethan English and Middle English, which is only separated by a century or two (I forget when he wrote the Canterbury Tales; Chaucer lived from 134(3? 1345?) to 1400, and the Tudor period starts in 1485ish, at the end of the Wars of the Roses). The difference from Chaucer to Shakespeare (1564-1616) is night and day (mostly because Chaucer is one of the first people who bothered to write fiction in English (yes, there are other examples, but few had the staying power of the Canterbury Tales, save for Beowulf, which isn't even pure Old English, as it's heavily influenced by West Saxon dialect).
I had a similar conversation with some of my old students who were working on time travel stories. One of them was really hung up on the minute details (he wanted to be a typical hard Sci-Fi writer) surrounding how far back the characters could go while still being understood (and understanding in return). We came up with a functional limit of about five hundred years without making serious effort (or a handwave-y gadget or other such nonsense; but again, he wanted to be a
hard Sci-Fi writer, so the whatsit couldn't just work, he had to be able to explain
how, but he also didn't want to include any nonsense materials or energy that could do it. Needless to say, he was never quite able to get out of his own way and just write the damn story).
Anywho, this was a long tangent that could've just been summarized by me simply stating my disagreement with the spirit of Weinreich's reductionist remark.