It will, nobody said Nintendo will stop selling DS when it launches 3DS and it wouldn't make any sense, most people won't buy 3DS at its launch price but a lot of people that won't buy a DS at its current price could buy it if it drops to around $100. DS has an immense and varied SW library that doesn't need 3D and that can sell the system also to people lukewarm towards it if the price is right.
Edit @Buzzi:
I think that if Nintendo wants to keep it alive long enough, and it could be the case if costs don't allow to cut 3DS price quickly to enlarge its audience, DS could easily reach even 180M, and if Nintendo goes further, following a "Sony approach", it could maybe reach even 200M.
I understand that DS' and Wii's success is so huge that Nintendo is maybe scared of becoming too tied to them (*), it will have to carefully evaluate pros and cons about it before deciding when killing them.
(*) but it must also be said that it wasn't PS2 success to hurt PS3, as it's also proven by PSOne not damaging PS2, instead they were just Sony's mistakes happening together with XB360 early launch and Wii's overwhelming success to hurt it.
