Every good show must have an end, right? Sometimes this end is hard to accept. And from this fact comes the creation of spinoffs.
For example, The Office has been widely accepted as one of the best and funniest shows today. But before the show had even finished filming, several spinoffs (including one focused on Dwight’s farm life and one focused on Andy) had already been announced. While these shows eventually came to nothing, this is a good opportunity to evaluate the role and merits of spinoffs and sequels in the media.
I personally believe that spinoffs have the power to ruin a show (or book, or movie) if done poorly. They can taint your previous recollection of characters and ideas, because you will tend to remember the last thing you see. Perhaps worse, if a spinoff is done with mediocrity, it tend to remind you that the beloved original was only really made for monetary value. And that, even if it means somewhat destroying the original thing, they will always pursue more cash.
Of course, it is important to see the other side of the issue. Sometimes sequels are perfectly acceptable. After all, the US office could definitely be considered a spinoff of the original UK series. And it was just as successful (if not more so) than the original series. And J.K. Rowling wrote seven full Harry Potter books and never lost following along the way.
In conclusion, it is important to recognize that sequels and spinoffs can be a valuable addition to a series. But it is also important to know that to preserve a successful show or movie, you must know when to stop.