the golden age of ds deving is over. there won't be a second generation. that generation is going to start with systems like the 3ds and vita. they may use the ds as a testing grounds to learn coding but not to make serious projects. just look back over the last 2 years and you will see how drastically the ds homebrew scene died off. this year we saw a small handful of updates to some really good projects but nothing new.
I think you're wrong to be honest. I'm still seeing new people take up DS homebrew as a hobby and stopping by the Blitzed IRC channels to ask questions. There are still people asking me about GBA related things & I thought that died years ago.
3DS if and when it gets opened will be a significantly more difficult and different machine to code for - GBA, DS and DSi are great little devices to program for and probably the last of the truly old skool consoles. Newer consoles (including the 3DS) have done away with tile based games and raster effects, they're all heavily 3D hardware oriented so they'll require a significantly more advanced skillset and I probably wouldn't advise a novice to start out trying to write games for these things.
Something else to consider is that there are still thriving ZX Spectrum and C64 scenes, there's no reason to suppose that the DS scene will just die, especially considering how cheap and easy it is to set yourself up with the means to run code on a DS.
Also, phones are crap - touchscreen buttons are *nothing* like the real thing.