The writer has a passion in mind, the sequel writer has cash in his eyes. Sequels, I think, are good if they are written by the same guy. After the writer passes away just let his work of art be his. Its okay to be inspired by it. But why not take it and put a different spin on it and then give it a totally new name.
All good ideas come from somewhere but its the shock and twist that leaves us wanting more. Give somebody their tell their story and end it as they want.
Most masterpieces are left on a satisfying note, and then the credits roll. There is no "the end..?" or "or is it...?", just "the end". But some movies do so well, that Hollywood believes that they can make lightning strike twice. I certainly don't think that ALL sequels are trash, but some just seem unnecessary and you can tell that the directors are out to make a quick buck by riding the last movie's success.
The Dark Knight? I think this is the best example of a sequel that eclipses almost entirely its previous installment. However, "films" like Transformers and that kind of stuff really kill the credibility that sequels could actually perform better than its respective predecessor. The best sequels are the ones that take certain elements from its previous installment and translate them in a deeper and better way and, ultimately, it could be generally better than the predecessor. Yes, some movies are practically ruined by their sequels (The Matrix for example) but others do not, such as, previously said, The Dark Knight. This movie in particular set a new era for superhero movies which, comparatively, are generally better than those before this 2008 success. Commercially, sequels help the movie industry in a huge way, but the quality has delicacy into them, so I think sequels (the ones that have great potential) must be encouraged to be done, and the best ones include these: an independent story that somewhat connects with the prequel, keep the character development in a believable way, and keep the spirit of the story. Keep them fresh!