New IPs can be huge successes. The problem is that they're risks. People like the familiar. They like familiar names, genres, developers (to a smaller extent). They tend to trust people around them before buying new things. Capcom recently had two good successes with Dead Rising and Lost Planet, although those games were fairly familiar in terms of their composition. There is almost always a danger of sinking a product by repeatedly slapping a name on it as long as that series retains the same fundamental gameplay. There is a diminishing aspect to it, and the problem with so many remakes and spinoffs is that they're generally trying to piggy back off of the success of the main series without trying to jump start it or revitilize it or become something bigger and better, and so it continues to sort of erode. There are only so many titles that can be released without moving forward, especially in such a short period of time.
New IPs can succeed depending on how they are handled. Quality sometimes does speak for itself. Banjo Kazooie was called the spiritual successor to Mario 64. But I think it was easily marketed. High quality game, developed by Rare, published by Nintendo, a standard platformer, perhaps the next greatest after Mario, and perhaps most important, it had a wide appeal. Not all new IPs are destined to become smashing successes. Not all ideas are going to be wide reaching. But I think success can be maximized if some steps are taken. The hardest thing is when you're trying to sell a new concept. But again, I think the most important aspect is having a concept that's easy to get. If your concept is kind of heavy handed or complex, then it might not fly with a huge number of people. And there are places for both games.
In Square's case, I'm still trying to figure out whether it's a fundamental problem with their plan. I think the first Kingdom Hearts was the last thing apart from the main Final Fantasy series that really set the videogame world on fire. Perhaps I just don't follow Square closely enough, so this is an honest question: have they done anything lately that's really new and interesting and successful all at the same time?