I'm not so sure that gave Dern a major boost. I've read several times that he received all kinds of death threats after "killing" the Duke.
As a role, although unpopular with JW fans it certainly got Dern noticed by the wide film making community.
Prior to The Cowboys he played very small supporting roles virtually being an extra with dialogue. After The Cowboys he landed two leading roles in Silent Running & Family Plot amazing when you look at his career before these films. He was unable to build on his raised profile and could not escape the psycho / nut roles that The Cowboys & Black Sunday typecast with as an actor.
