Yeah, the 3DS comes out in a year. That gives the DSi something in the region of two years total lifespan before it is made obsolete. The point is that the DSi is twice as fast as previous models, has four times the amount of RAM.. etc. We will never see these specs really used, since nobody is going to make real DSi software any more.
Why would they? It won't play on the millions and millions of DS/DSL consoles, and if they were going to make something just for DSi, they may as well make something for 3DS. I think you're painting the picture prettier than it is. There is something like four actual DSi games, and the thing has been out over a year (!). There are far more games for the unreleased 3DS than there are for the year-old DSi.
In my opinion, this is because developers are no-longer interested in the DSi. There is only DS or 3DS. Some people (even very recently) will have paid the full �£130 or whatever for a DSi. They might as well have just bought a DSL. They'll only ever play normal DS games on it, the only DSi software that anyone's really going to get is the crap on the online shop.
What I'm saying here is that the DSi hasn't really been tapped, and probably never will because of the looming 3DS. The devs of all those 3DS games have probably been beavering away at them for a year, or at least six months. Hence the lack of real DSi stuff.
But hey - maybe Nintendo will keep me happy with some faceplates next year or one or two DSiWare offerings? I bet we see less than five real DSi games in the next year. Lulz.