I should have phrased this as a question, instead of being dismissively declarative.
>If, upon hearing that "ripgrep can replace grep," you actually hear, "ripgrep can be used in every instance grep can be used, in exactly the same way, for the same use cases, with exactly the same bug-for-bug behavior," then no, ripgrep trivially cannot replace grep. Moreover, ripgrep will never replace grep. If, upon hearing that "ripgrep can replace grep," you actually hear, "ripgrep can replace grep in some cases and not in other use cases," then yes, that is indeed true!
Yes, it's a persistent misunderstanding because communication is hard and folks aren't always exactly precise. It is very common to hear from someone, "ripgrep has replaced grep for me." You might even here people state it more objectively, like, "ripgrep is a grep replacement." The problem is that the word "replace" or "replacement" means different things to different people. So that FAQ item was meant to tease those meanings apart.
>If, upon hearing that "ripgrep can replace grep," you actually hear, "ripgrep can be used in every instance grep can be used, in exactly the same way, for the same use cases, with exactly the same bug-for-bug behavior," then no, ripgrep trivially cannot replace grep. Moreover, ripgrep will never replace grep. If, upon hearing that "ripgrep can replace grep," you actually hear, "ripgrep can replace grep in some cases and not in other use cases," then yes, that is indeed true!
I think this statement says it all.