I have a theory that, the reason "you" survived but "thou" didn't, is because when the printing press became a thing, Y was used as a substitute for "thorn." Which is why we have things like Ye olde, which should be read as "the old." Since Y was being used for the "th," thou and you both ended up as "you."
Heya heya! :) While I was not a contributor to the post you’re responding to, I can make some responses to it! I can’t be too thorough given as life is terrible and busy, but hey, hopefully it’s still factual enough woot! And language is cool. It’s an interesting idea tying the word change into the printing press.
I’m not sure how familiar you are with linguistics, so hopefully I’m not saying stuff you already know, especially since you are familiar with the whole you-versus-thou history a bit… but here are my thoughts. Diachronic linguists have pretty thoroughly combed through “thou” and the reasons why it became obsolete in favor of “you.”
The use of “you” being used as a polite form to address a single person started transitioning following the Norman conquest of England in the eleventh century. By 1600, “you” had become so popular that “thou” and “thee” were basically only used in intimate or rude contexts. By the end of the 1600s, “you” was normal in essentially all contexts while “thou” was almost exclusively seen in religious contexts or regional dialects.
For instance, if we look at Early Modern English writing, like the love letters of Dorothy Osborn (1627-1695) to William Temple, even in this setting to her beloved, talking to one person, she always uses “you”:
You bid me write every week, and I am doing it without considering how it will come to you. Let Nan look to that, with whom, I suppose, you have left the orders of conveyance. I have your last letter; but Jane, to whom you refer me, is not yet come down. On Tuesday I expect her; and if she be not engaged, I shall give her no cause hereafter to believe that she is a burden to me, though I have no employment for her but that of talking to me when I am in the humour of saying nothing. Your dog is come too, and I have received him with all the kindness that is due to anything you send; have defended him from the envy and malice of a troop of greyhounds that used to be in favour with me; and he is so sensible of my care over him, that he is pleased with nobody else, and follows me as if we had been of long acquaintance. ‘Tis well you are gone past my recovery. My heart has failed me twenty times since you went, and, had you been within my call, I had brought you back as often, through I know thirty miles’ distance and three hundred are the same thing. You will be so kind, I am sure, as to write back by the coach and tell me what the success of your journey so far has been. After that, I expect no more (unless you stay for a wind) till you arrive at Dublin.
Now, since the printing press was invented in 1440, technically that would be during some part of the transition period from “you” to “thou.” I also have tried to do hurried research on the historic pronunciations of both “you” and “thou” - after all, their similar spellings suggest a past rhyme. English spelling used to be more closely aligned to actual pronunciation, and part of the reason things got quirky was due to the Great Vowel Shift. The Great Vowel Shift basically had all of the vowels change pronunciations in English from what they were in the past. In this shift, a long “oo” vowel /u:/ shifted to the “ow” diphthong /aʊ/, as in how we now pronounce “house.” From some information I’ve seen in the Oxford English Dictionary, it looks like “you” could have undergone the shift to /aʊ/ as well, and was pronounced that way in the 1500s and 1600s during the time that the printing press would have started making its rounds into the English language, with Caxton introducing the printing press to England in 1476 CE (apparently in the later half of the 1600s pronouncing “you” like “yow” became uncool, and hence it never kept up with the lasting Great Vowel Shift splendor?). So all of this seems to line up initially with your idea.
Unfortunately, though, it doesn’t look like this printing press idea would be a likely scenario.
Language change tends to be driven through speech, not writing - makes sense, given as writing is not developed in every language culture, and until very recently literacy was not widespread in most populations. Language change also tends to be driven by certain demographics, notably young women. However, during the time the printing press was a new deal, illiteracy was extraordinarily high - it looks like 90% of English men in 1500 CE couldn’t read, and even by 1700, male literacy was only at 40%. Illiteracy in women is notably higher. The demographic most likely to build change from “thou” to “you” wouldn’t have been able to read the text on the printing press that used “y” instead of the thorn. Yes, people would have wanted to mimic the upper class, the people more likely to read, but spreading this pronunciation around to the masses would be pretty problematic.
I also thiiiiiiiink that, while “ye” for “the” certainly happened, the thorn was already fading away from English orthography, with “th” gaining popularity even in the 1300s, or something around there. I’m not sure about this as some other points I’m making here, though… I don’t tend to study orthography, cool as it is.
Last, what happened with “thou” versus “you” is actually a really common language change cross-linguistically. Theoretically it’s very strong to stick to patterns that we see across languages, notably even across unrelated languages. Using a plural form of a pronoun is a very common designation of respect. For instance, this occurs in Croatian with ti (singular & informal) versus vi (plural/formal), in Kannada with neenu (singular & informal) versus neevu (plural/formal), in Turkish with sen (singular & informal) versus siz (plural/formal), in French with tu (singular & informal) versus vous (plural/formal), and in Tamil regarding the use of niinka. There’s also the phenomenon of the royal plural (ex: Hebrew), where a plural pronoun is used as a sign of respect to a single person holding a high position of leadership. So when it comes to tracing back to what English did regarding “thou” versus “you,” it’s completely understandable to align with a theory that patterns after many of the world’s languages.