Wrestlers can adjust their actions according to who they face in the ring for one night, but ultimately he must have a clear identity and the easiest way for fans to identify with him is to make a distinction of whether he is a good guy or bad guy. Take away that and you are basically forcing every fan to follow your product every week or don't bother watching. Sure the ideal thing is every fan is loyal to the product every week but that isn't reality.
Good triumphs over Evil is what people pay to see in theatre. Take away the roles and all you have is just 'sports' and we see how far that concept went in pro-wrestling. If all you want to watch is sports and fighting, MMA/Boxing will appeal more to you than pro-wrestling. Why should WWE make a product to cater to people like that who will always deem it as a inferior when they can make their own brand of 'fighting' which the fans will regard as the best?
Good triumphs over Evil is what people pay to see in theatre. Take away the roles and all you have is just 'sports' and we see how far that concept went in pro-wrestling. If all you want to watch is sports and fighting, MMA/Boxing will appeal more to you than pro-wrestling. Why should WWE make a product to cater to people like that who will always deem it as a inferior when they can make their own brand of 'fighting' which the fans will regard as the best?