If I were to break down your criticism into the points you’re trying to make, I’d summarize it as follows:
You find Github’s Markdown renderer to be more readable than Asciidoctor’s HTML.
You find it harder to contribute because of… well, it’s not really clear. I’ll come back to that.
Personally, I find Github’s renderer to be… adequate. If you just need to drop some text on people, it’s fine. But it’s nothing to write home about.
Font selection? I mean sure, I’m personally not a big fan of serif fonts on a screen. And the one they choose is very, vertical. which is weird. But it’s still better than Github’s “font selection” that seems to consist of “whatever your browser uses”.
You may not like the TOC on the side, but it make it so much easier to actually find things. To be in one part of the document, and need to jump somewhere without having to reach for a keyboard to go all the way back to the top. Yes, I wish there were a way to have a toggle button to be able to turn it on/off as needed.
Then there’s a matter of readability you neglect: printing. Github’s renderer has no print functionality; it’s just whatever your browser spits out. That may be adequate to some degree, but it’s hardly professional. And yes, being able to print stuff out and read it offline still matters.
Overall, I don’t find the readability of the document to have been compromised. Yes, individual people will have personal preferences, but I don’t see anything significantly outside of personal feelings with your argument here.
This is an argument I don’t understand at all. You claim that “a user needs to figure out that now source and doc are not the same thing”. But that was always the case. It’s not like you’re looking at the raw Markdown when you’re looking at the glTF specification. It’s a rendering of it.
The only difference is, I guess, that you can’t preview it in Github. Well… so what? There are dozens of preview tools out there for Asciidoctor. There’s a perfectly adequate plugin for Visual Studio Code that can handle it just fine. You need not “build the whole thing just to see or make a change”.
Also, “learn asciidoc” is not exactly a Herculean task. Asciidoctor is pretty readable markup. It’s different from Markdown, but Markdown is different from other text markup languages. You can look at an Asciidoctor file and have a pretty good idea what’s going on. If you need to make a few wording corrections or whatever, it should be no problem to just edit the files and fix the words.
And I have no idea what you’re referring to with “while in the past, you could even compare different versions of the doc by simply looking at the commit history.” You can still do that now. Asciidoctor isn’t binary; it’s perfectly readable and entirely diff-able text. No different from Markdown.
Overall, I feel like you should provide clearer examples of the problems if they are going to be corrected.