The main reason most golfers have a slice ingrained is because they come over the top. This is the root of the problem. The fact that the face of the club is typically left open is not the cause of the problem, but rather the logical compensation the golfer makes to be able to play with their outside to inside swing path. If they closed the club face, by rolling the wrists or by any other method, they would pull the ball straight left or worse yet hook it hard left off the course.

Yes, there are a few golfers who will push-slice the ball on occasion, or while they are learning, but not many people consistently play with a push-slice. As the OP points out, this can be fixed rather easily with a club face angle correction. Unfortunately, most slicers are not pushing the ball, but rather coming OTT. This is not so easily fixed, as we hear oh so often around here.