Again, they make new series because they have to. They would love to have Nintendo's stable of recognizable characters, but they don't so they have to create their own like Nintendo once had to. These companies aren't creating new franchises because they're more creative than Nintendo. And as we've seen, once they launch a successful one, they bleed them dry no matter how little it makes sense for the series. By last count, Ubisoft has released 41 Assassin's Creed games since the series launched in 2007. God of War's story arc ended with III over three years ago yet Sony still managed to release two additional games in the series.
Yes, Nintendo relies on their characters a little too much sometimes. As previously mentioned, Kirby's Epic Yarn is hardly a Kirby game. It even began as a completely original intellectual property before Nintendo asked Good Feel to change it. However, people are way too attached to this idea that companies, namely Nintendo, need to come up with new franchises for the newness of it.
Hypothetically, if the Galaxy team decides to make a Mario-esque game that is not a Mario game just so it's not a Mario game, consider why it shouldn't just be a Mario game. Why create something similar to Mario and try to replicate that level of success when you can just make a Mario game? Mario's universe is so open and inconsistent that Nintendo just does whatever they want in it. There are no rules. He can swim forever in one game, but has limited oxygen in another. He has a spacesuit in Super Mario Land, but not in Super Mario Galaxy. If you really think Nintendo needs to create a new franchise because they're too limited by the series to present new, innovate ideas, you haven't played enough Mario games. Nintendo has established the series as one where anything goes, one that constantly changes depending on what they want it to be depending on the game. It just so happens to be their most popular intellectual property.
Now, I'm not saying Nintendo shouldn't look to create new franchises. They should when appropriate. I think there has to be a balance. Creating a new franchise should make sense. What is the developer trying to accomplish? If they have to betray an entire character just to fit it into the series (e.g. Kirby's Epic Yarn), it should probably be its own franchise. I advocate Retro Studios creating a new franchise so long as they're creating something that Nintendo doesn't already have a hand in. If it means Retro Studios makes a first-person shooter, it makes sense to explore that because Nintendo has no first-person shooter franchises of their own and they're not getting nearly as much support in that area from third parties. However, if Retro Studios is basically making Metroid Prime without Samus, I really don't see the point. Yeah, it's new and may catch the attention of people who aren't Metroid fans since it lacks Metroid in the title, but I don't think that should be the only or even main reason. If it has to be new, really commit to it.