No cache is optimal for all use cases. That's an impossible goal.

Thus why things like Freqfs exist and we don't always "let the OS take care of this."

Yea friend, we're in violent agreement :)

the actual question is whether the person making the choice is up to the task of measuring/proving that their manual caching is optimal for their use case. A lot of the time the answer is simply "no". For example, the people who say "just buy enough RAM and turn virtual memory off" for the most part do not understand the implications of what they are talking about.