If you are saying that if Duke hadn't hired him, he wouldn't have accomplished much and might have died alone in PA, well, that's impossible to dispute. Obviously no one would know him if Duke hadn't hired him.
The bigger question is, what would the Ellington-Strayhorn songs sound like if Duke hadn't hired him? Obviously we would still celebrate Duke - but that doesn't mean that we shouldn't celebrate Strayhorn as well.
Strayhorn is fine and I celebrate him.
Ellington's reputation was established earlier.