I think there's plenty of years left in the DS line. I still play my Dsi XL even though I have a 3DS. There are still plenty of games scheduled for release on the system - Captain Morgane and the Golden Turtle for example.
Some people are waiting for a 3DS XL to be released, wanting a larger touch screen than the current 3DS has.
I will be playing my DSi for many years to come and there will be software support as well.