I'd say Charles Rosen is talking out of his backside.
It's one of the most ridiculous concepts I've ever heard.
"Once played, they become public property." Don't have a problem with that. The public can do what it likes with it and the author/ composer doesn't even have choice in the matter. You can sit down and watch a movie and think "that movie was about THIS". The screen writer may not have wanted you to think that, nor did the director, but, you do. Nothing they can do about it.
"That is why we can maintain that a composer does not always knows how best to interpret his own work."
This statement reminds me of something that comes out of my next door neighbour's dogs rear end. In fact I think the dog produces something more substantial.
Just because someone doesn't 'get' what the director/writer was aiming at, doesn't mean the director'writer didn't understand what he wanted.
If I give hints to my wife that I want a ferrari for Christmas and she goes out and buys me a motorised model, I'm not going to admit - oh yeah, THATS what I meant - boy luv, you sure know me better than I know myself!
"..he cannot control future performances" Of course. No problem with this statement.
"his opinion on how to play it ... not absolutely privelaged" If it was, he shouldn't have published it in the first place.
"we must ...admit that very often the composer.. does not fully understand his own intentions." Crap. Pretentious, arrogant, presumptuous and irrelevent. "VERY OFTEN the composer..." please! Who is he talking about - if it's his wife, brother, someone else he knows, then fine. If he's trying to make that assertion with Beethoven, Mozart; DEAD people, then he has no credibility to his statement unless he owns a ouija board.