I think it's fair to attribute the result to both the writer and the actor. You're right that Stewart could've sold a morally compromised captain, but equally so he brought something to the Picard role that other actors would not have necessarily brought. One can commonly see in theatre how with the same vision by the playwright, two actors can turn out wildly different results for that character. The same would have happened to Picard if he had been played by someone else.