> Even if you "know" all Kanji in a word you'll likely not understand the word's meaning unless it's something simple and concrete.
I don't think anyone passingly familiar with Japanese thinks otherwise. That is, I think you're arguing against a position here that nobody actually holds. The argument for memorizing kanji is that it makes it easier to learn compound words, not that you'll just know them without learning them.
I agree that people with some Japanese knowledge likely already know this. But out there you will find a lot of marketing material/posts targeting absolute beginners promoting some kind of "Top X% Kanji lists", as if they are a huge shortcut and secret to quickly learning Japanese. So I'm just saying they aren't.
If you talk to people who don't know much about Japanese they often believe that memorizing the characters is the difficult part and doing so will help you understand a large fraction of written text. I think it's quite a common misconception.
I feel like a lot of this perception comes from a tendency for people to equate Kanji characters with words. I always explain to others that a character is kind of like a root, e.g. Sub-optimus-al = suboptimal. It would be crazy to say one can learn English by memorizing just a few hundred Latin roots and suffixes, and in fact such a claim is so irrelevant nobody even keeps track of the statistics.
I don't think anyone passingly familiar with Japanese thinks otherwise. That is, I think you're arguing against a position here that nobody actually holds. The argument for memorizing kanji is that it makes it easier to learn compound words, not that you'll just know them without learning them.