Just like the worlds of film, TV shows and games, anime has it's top-tier brands. There are series' and characters that anime lovers have made more well-known than other content out there. It's inevitable that some anime content rises above the rest and really gets some big attention from your everyday person. These franchises become well known and loved by many as the years continue on.
Eventually, the main series that started all the love will end its run. All good things come to an end...but that doesn't mean there's not a new beginning just around the corner. If there's one thing all creative industries like to do, it's make sequels. Working within the confines of an established brand is much easier than starting from scratch. You've already got a built-in fan base hunting for new content.
Sometimes you have to wonder if that's the right decision though. No one wants to see the good name of a series tarnished by a lackluster spin-off or sequel. Luckily with anime, most future content based on an existing IP is good. Take a look at what happened with Ghost in the Shell. The original movie spawned another film as well as a great TV series. In this instance, continuing the brand was a smart move.
Like I said, that's not always the case. Fans, at one time or another, have found themselves asking why the anime industry decided to further a story when all was said and done the firs time around. How do you guys feel about continuing anime franchises after their initial run? Should most series' get a chance at Robotech-like runs, or should we see more features taking the Fooly Cooly route?