Compelling software and even the early adopter syndrome sells hardware. Backwards compatibility is not a huge motivator; it's an extra. How many people were motivated to buy the Wii because it could play Gamecube games? That's ridiculous. Thank Wii Sports and Twilight Princess. No one is kicking down Sony's door to reinstate 2 whole generation's worth of backwards compatibility in PS3 and games are still released on PS2. So, I don't think you can make that claim. People buy new hardware primarily on the merits of the system itself, not its ability to play legacy games. It's a nice feature, sure, but it's hardly a deal breaker. Since I already own a DSi, I'd take a cheaper, smaller 3DS if Nintendo ditched backwards compatibility. In every backwards compatibility enabled console's life, there comes a point when people stop caring about it. By the time Microsoft cut Halo 2 from Xbox Live, like 7 people were still playing it.
Additionally, I kind of feel like backwards compatibility encourages laziness. New hardware is rushed out the door with minimal games because the idea is that people can play their old games. Sony PR'd the f*ck out of that in 2000 and it's still bullsh*t. PS2 had a god awful launch. So did DS. People will still have the old systems. Or they should and thus will still be able to play the old games. They could get $20 in trade-in value, but then you encourage used hardware/software sales and lose money that way. Launch with a brand new Super Mario game then see how much people give a damn about old DS games. Even if backwards compatibility is there, Nintendo should be giving people a legitimate reason to buy 3DS, not a more expensive DS for a while until the real 3DS games come out.