Why does String + &str exist then? It's a just a function (and please don't point "other mainstream languages": this is exactly the kind widespread insonsistencies that would deserve to get done right at least once). Anyway I agree with @mjbshaw: this topic has already been discussed many time and certainly a RFC is the only productive way to make it move forward.
I would like to point this post to @ibraheemdev where I tried to show that more generally the Rust operator story as a whole would need clarification if we want to have a constructive discussion.