I see DS as an experiment. You see if DS takes off, then it would become the viable replacement to the GBA. If it doesn't take off, they always have the normal GB line.
If DS does take off, the way we play games on a handheld could possibly be changed forever as it would allow games ideas that have never been possible or effective on a single screen to be made and played. If the DS is to replace the GB line, we will still have our normal games and what not with the second screen not used at all, or in some limited fashion. Or maybe by not using the second screen, one could pump the power of both processors into one screen.
Whether it follows the orginal GB line or improvements is generally pointless because graphically, it is almost a given that it will have better graphics than a GBA. It is Nintendo hedging it's bets and if the DS becomes the primary handheld, so much the better for gamers.
But one could also think of this as the GBA version of the GBC, an upgrade, but not much of one. In that case, maybe they should come back and try later when the gap become more siginficant.