Whaaat? The quality of a game can slip...the quality of anything can slip. The Sonic series is a popular example. Donkey Kong 64, Super Mario Sunshine, Star Fox Adventures, Castlevania 64....there are players out there who will argue that all of these games represent a slip in quality, and in my opinion they are right (even though I still think those are all good or excellent games).
But yes, the slip can also be from the viewer's perspective. In this case, I think it's wise to follow that old saying, "Leave them wanting more". I don't really want more of a lot of Nintendo's franchises, because they're already making more than I can handle. Mario Party is a great example. I'm interested in Mario Party 6 because of the new mic games, but there's a voice in the back of my head saying, "There will be another one next year, spend your money on something else." Taken all on it's own, Mario Party 6 will be a great game, I don't think anyone who loves multiplayer games should be without a Mario Party game. But added to the 5 other games in the series, it's just not that special.
Edit: as for the new screenshots, they're not perfect, but they're still sweet. I don't think we'll have time to worry about the quality of textures on that monster as he kicks our skinny butts.