I'm Japanese, but I don't know "汗牛充棟, 白河夜船". I thought I know 朝三暮四, but I google it now and I was wrong

Then you see what I mean when you say you often cannot guess the meaning of yojijukugo. 一方では、「焼肉定食」や「全席禁煙」は読むだけでその意味を把握することができますが、他方では「比翼連理」や「明鏡止水」のように明確な典拠を見出せないものもあります。

Like I said, there are a few yojijukugo that everyone knows like 弱肉強食 or 一石二鳥 or 一目瞭然, but most are obscure even if they are written with 常用漢字. Every yojijukugo I have mentioned in this thread are solely composed of 常用漢字. A big reason for the obscurity is many of them are relics that refer to Buddhism, old Chinese history, or old Japanese history.

So yes, I would agree that since so many yojijukugo are rare, most learners can safely ignore them. I only know a good amount of yojijukugo because they are tested on the 漢字検定.

Most Japanese words are the combinations of 常用漢字. Japanese people can guess most of them(and not guessable words are not known and dying).

Really? You think words like 蛍雪 and 亀裂 are not known and dying? 　 I agree that a lot of words are guessable if you look at the component kanji. In fact, I'd say the majority are guessable by looking at the kanji. But, a lot of words are not guessable, like 人口. A person's mouth? No, "population". You just don't think about it because it is just a common word to you. It is just like how (most) English speakers don't think about the composition of words like "hysteria".