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.)