Couldn’t agree more. Interesting, my interpretation and response to your original comment was, in a way, the opposite of what you meant. Sorry about that. Also interesting because my example, Brahms “Clarinet Sonata” No. 1 serves as a perfect example of what you meant.
Brahms dedicated his two clarinet sonatas to the great clarinetist Richard Muhlfeld who Brahms had heard and been so impressed by that he had previously written his Clarinet Trio for him. Brahms was said to have “discovered the beauty of sound of the clarinet”. When he composed those pieces Brahms expressed his voice with the sound of the clarinet in mind. As you say, what he had to say (his voice) would not have been expressed as well on any other instrument. Perfect example:
I suppose in part because relatively few major works have been composed for the viola, Brahms himself later transcribed the two sonatas with the clarinet part adapted for viola. Still beautiful music; but, I don’t hear as much of his voice when performed that way.
https://youtu.be/Vi8q54R7ObA