Well, I'll give a brief response now, and let it all percolate in my brain for a while since I'm riding insomnia at this point.

The first thing I notice is that you've got a misconception about how languages as they exist came about. Thou/thee to you was a gradual transition. The words used before that were also a gradual transition into thee/thou and other archaic english pronouns.

Every word in every language currently spoken evolved into what they are now, with possible exceptions in isolated languages I haven't run across.

A neopronoun, by the only definition I'm aware of, is a coined word, it's created whole cloth, or intentionally changed from an existing word. Thus, "you" has never been a neopronoun unless you're stretching the term for colloquial usage. Which is fine, but it still negates the idea that current pronouns in common usage were once new. They weren't, they just had pronunciations shifted over time as languages do.

Secondly, I said that neopronouns impair language acquisition because they add a layer of complexity that doesn't (typically) match established grammar and usage. By that, I mean that if a pre-verbal child is picking up language from people around them, which is the normative way, the more complex the structure of pronouns, the longer it takes to have the full vocabulary of them, and to understand how they're used.

Which is where I have an initial thought. Neopronouns could fit in fine if there's a limited number, but individualized pronouns would greatly complicate early language acquisition, and even early education, though once you reach the stage of formalized education, I dunno that complexity is a good or bad thing, but it does make it take longer and more difficult for the students.

I still say that individual pronouns is an utter fail because it breaks too many social contracts, and requires constant shifting of language in a confusing and energy wasting way. As an example; if I am talking to my friend Bob about drag, then I have to either use drag both as the nickname and pronoun, or choose to ignore it as a pronoun entirely for the sake of Bob being able to understand the sentences. Now, not everyone using individual pronouns made the choice of using one that's the diminutive of their name (and you may not have done so offline, but you aren't going by your legal name here, so it's irrelevant). So, for them it's less of a cognitive clusterfuck than what drag runs into on lemmy.

But even when you've got someone named bigdog, and uses pup as their individual pronoun, the problem exists that there's extra steps placed in between the two people talking. You have to take the time every time, to set up the frame of reference. As a thought experiment, that's great fun.

But it's an utter fail in practice because it's not something a consensus of people will be willing to do. It just isn't. It would require convincing not even just those that actively oppose it, but even allies that are just exhausted with it, and the vast majority that don't care enough at all. Convincing that many people to rebuild not only their own language usage, but formal education as well, all to the dubious benefit of a minority that can't even agree on a consistent and reliable way to go about using individual pronouns, it won't happen.

So it remains a thought experiment. An exercise.

Remember, all the shifts from "thee" , "thou", and"you" to just "you", and the ones that led up to that, were shaped by consensus. You had prior words that turned into thee and you. Then, people gradually stopped using thee as first person, thou as 2nd person singular and you as 2nd person plural, and switched to just using "you" for all three. Well, that's what my memory says each word was; I could have switched up which were singular vs plural, but I think you get the point that it was a lot of people changing words they were already using, not replacing those words with new ones.

But you won't hear me argue that language can't change. Any language being spoken actively by people will change. It will even change via writing, though it would be slower since we tend to stick to more formal, codified grammar and vocabulary since the age of print came about. My argument is that any given specific change has to pass muster as useful and acceptable to enough people before it happens, and neuter neither neopronouns, nor individual pronouns have a snowball's chance of that happening.

There's also an off topic side issue that it's not the right time to fight for it, but that's definitely off topic.