Doctor Who is a show who made a great success during its first run, died out and them was reborn to new success. Why? Because its creator decided that it was needed to change in order to attract a brand new audience, while the old one was, well, dying, literally. As many of the old fans were getting old, there was a need to attract a new audience, many of whom never really saw a Doctor Who episode. And it worked.
It is not just the British who find a nice solution to avoid a TV show stale by changing its main actor from time to time, keeping some freshness in the series. Japanese shows like Kamen Rider and the Super Sentai series also found a solution to the same problem.
They only let the saga go for 51 (may vary a little) episodes and when the time to do a new season, they just rebooted the whole thing. New heroes, villains, themes, plot. everything was renewed at once, while keeping the elements that made the previous season successful and getting rid of anything people didn't liked. By doing this, they avoided stretching too much the shows, tiring the character or changing a beloved character into something people don't liked.
Take Peter Parker and his Spider-man. He have being around during decades. And have suffered a lot from it. New writers trying to give a new direction, or to update the character to modern times, or trying to change him significantly. And fans hated it. because they already have a image of Peter Parker from decades of stories, any attempt in changing him would be met with criticism, no matter how good the changes were. it was just not Peter Parker anymore.
Americans try to reboot its heroes from time to time, but usually met difficulties. Because they try to apply those changes to existing characters, or trying to make new characters carry the legacy of a previous one without letting the previous one go. But sometimes they do certain things who work.
By not just recycling Captain Kirk and his crew, but by allowing other settings, cast and themes, Star Trek and its several shows reached a long lived run. Because they let the old rest and the new do their own thing, they was able to make it better.
Sometimes, shows, games, comics and any media who have a long run need to reboot itself. If they don't do it, they will become tiresome, repetitive. Or if their creators try to change it, the long time fans will feel betrayed, or that the show lost its spirit. Allowing change, renewal on them can keep them fresh, attract new public without losing or angering the old fans. It is needed a lot of courage to do that, but the other option may be seeing a beloved franchise disappear slowly and painfully.
What don't change are due do disappear.