You know, there are some series where they should have stopped while they were ahead, such as the Crash Bandicoot games. 1, 2 and 3 were excellent, but ever since Naughty Dog left, they are very forgettable.
However, I'm glad they chose to continue the Metroid series. And I really hope the keep making them, at least until they've stretched the story and gameplay to its best. I say they should end the story at Metroid 6, and maybe make a few prequels or sequels, or even remakes. However, they should stop before they exhaust the series and maybe, as someone said, focus on a different part of the universe. Maybe the adventures of Kanden or something,
As a side note, I also think Nintendo should get around to creating a new character. Sure, I love to see the olden goldies going strong, but some new characters are needed, and not just new characters in games, like a new kirby sidekick. I mean a whole new franchise, with some sweet characters. create someone who can stand alongside Link and Samus as cool.
Also: ALPHA METROID! PRIME! WHERE!?