Well, to be honest I think the problem is that Mario is, and always has been, basically a dressed up version of "Jumpman" from platforms like the Commodore 64. Over the decades Mario has really taken the basic jump-oriented platformer idea and ran with it in some crazy directions, the best games are to the point where it's very easy to forget the humble origins and the simplicity of the concept even for those that were there. The thing is that in the end you can only do so much with this basic kind of game, and truthfully Mario did more with it than I would have thought possible, but I think they have reached the limit. It's all variations on things you've seen before, but that's because there really isn't anything left to do within the rules of the franchise. Coming up with something new that works for Mario isn't as easy as you might think. Mario at it's core being flashy, and having depth, but also being extremely casual in that it's approachable to nearly anyone, with the masters being able to really showboat and do all kinds of insane things with it. The problem with most fan ideas coming from masterful Mario players (to get to the point of being bored, and having seen everything to be disappointment) is that a lot of the things they think up wouldn't really be approachable to the basic casual player. Sure you could turn Mario into "I want to be the guy" and get really sadistic with it, and some people would like that, but it wouldn't really be Mario anymore at that point.
I'll also say that a lot of the problem is that we're dealing with a gaming generation here. It's kind of exceptional that there are even franchise that have run 20+ years for this to be an issue. A guy old enough to have played SMB when it first came out on a Nintendo and then played most of the games since, is probably not considering how the newer games look to younger, less jaded gamers. Oddly the serious "hardcore fans" who have followed a franchise like this from the beginning are by becoming hardcore fans, no longer the intended audience. You can't cater to a jaded 35-40 year old veteran while retaining the same casual innocence very well, and truthfully it's bloody amazing how many times they have managed to walk that line as it is.
I do not think a lot of these other titles like "Meat Boy" and others are really better or more innovative than Mario, they are just new, and when your getting tired of something, or frustrated, people tend to wander to the fresh, similar products, and scream their praises a bit too loudly in compared to the originals.
In short, I'm not a huge Mario fan, but I think he's doing fine as things are now, as long as there are new gamers coming in, and parents willing to get products like this for their kids, Mario will continue to have a prescence. Arguably Mario is in a position where he can thrive through consistincy without any innovation at all, which is unusual, so it's pretty good that we see it holding up this well.
Would it be nice to see more from Mario? Well yes, but to be honest I think that involves going into other, but similar, generes, something that seems to have failed,in initial attempts, so Nintendo hasn't stuck with it. I seem to remember games like "Mario Sunshine" getting some decent reviews, but being commercial failures, the fans just not really being there for the games.
This is my opinion at any rate.
