Frankly, I think it's great character development for Luke. As the saying goes, if a man still thinks and believes at 50, what he thought and believed at 20, he has wasted 30 years of his life. When we are young, are are optimistic about life and where we are going in the world. The older we get, the more reality smacks us in the face and realize it's not as happy go lucky as we thought, and we can become cynical. If the Sith or Empire keeps coming back no matter what Luke does after 30 years, and his even star pupil has turned against him, I would imagine he would become wholly depressed and live the life of a hermit. Also, he was living on the opposite side of the universe, it's not like he could take a walk down the street and say "here I am", like he was living next door to the rebellion's headquarters.
As for Lucas's universe, please, let's stop giving him so much credit. The man simply took the concept of Flash Gordon and Forbidden Planet, mixed in some Hidden Fortress, filmed a bunch of footage (while his producer was doing things like giving the actors character motivation), and the movie we got was the product of lots of great editing. Then other artists took over, and improved the universe to standards that Lucas didn't even think of. Then after 20 years of being kissed up to, Lucas grew a giant head, thought anything he touched was gold, and behold, we get the prequels, which are only good for making parody memes out of. Ironically, Ron Howard was originally asked to direct the prequels. If Lucas just gave Howard his ideas, and allowed him or someone else to write or direct, we would have gotten a far superior product. Fact of the matter is that the worst Star Wars movies are the ones where Lucas had total creative control over. So I don't feel bad if a new Star Wars movies doesn't "fit in Lucas' universe", because he truly never had a concept of what he was doing.
