[4bpp]

Japanese is not really the right example, because unlike Chinese the writing is really more phonetic than not (and when it isn't, as in kanbun texts, it's basically an idiosyncratic translation to a completely different language). I imagine that the reasonably educated Chinese should be able to read something like the https://en.m.wikipedia.org/wiki/Xi%27an_Stele from the 8th century just fine, given the regular typeface. English, from the same time period, has Beowulf, which is incomprehensible to any lay reader.

Once calligraphy/handwriting is involved, the situation on the Western side is not much better either. Modern Anglosphere children probably would struggle with 19th-century cursive like https://www.pinterest.com/pin/375276581427478862/ ; in Germanic countries, the handwriting system underwent deeper changes, so nobody apart from selected nerds and antiquarians can read Kurrent as in Goethe's letters - https://commons.m.wikimedia.org/wiki/File:Goethe_Brief_(nich... - or even the newer Sütterlin. Contra what some posters here claim, Roman cursive (https://en.m.wikipedia.org/wiki/Roman_cursive) is right out. I don't think this should be conflated with the question of whether the writing system is understood by future readers - as an imperfect computer analogy, an ASCII text document is in some meaningful sense more futureproof than an Autodesk Animator .FLI, even if the former is on a five-inch floppy and the latter is on a USB thumb drive.

(As for the effects of the Japan's Chinese character simplification, I think they are a bit overstated. I accidentally bought a 旧字体 copy of Mishima's Haru no Yuki at a book sale once, and at least as an L2 speaker I didn't find it particularly more painful to read than I find unmodernised Shakespeare as an L2 speaker of English.)

[shiroiushi]

The OP never said "Chinese", he said "eastern". Japanese and Korean are both eastern.

[4bpp]

That's a fair objection, but given the context I assume he was not actually having anything other than Chinese in mind. Otherwise one might as well bring up Vietnamese, which has a similar history and lexical makeup as JP/KR and is spoken about as far east, but is now written phonetically in Latin-based script with hardly any readers of the old Chinese-based writing system remaining.

[shiroiushi]

Yes, Vietnamese is another great objection, as it's similar to Korean that way. Given that China, Korea, Japan, and Vietnam are the 4 major "eastern" countries then, and only 1 of those might be true for his claim, then why did he say "eastern" instead of just "Chinese"? That's like claiming the American continents (North and South) use English because it's used in the United States. People in Quebec and almost every place south of Mexico will be quite offended, rightfully so.

[lucidguppy]

Yeah - that was my mistake.