At this point I'm not sure those sequels would sell more systems, even though I'm sure they'd sell amazingly well. The thing is, if people weren't drawn in by NSMB, why would they buy a DS for NSMB2? After a certain point in a system's lifecycle it's hard to say what the "system sellers" will be, since you expect that they've captured a large part of their intended market. Like, with Dragon Quest IX, I'd expect there might be a few DQ fans who haven't already bought a DS for DQM:J, DQIV, DQV, etc., but that group must be growing smaller every day...so who knows if there'll be that much of a HW bump when DQIX releases unless they come out with a special edition?
The DS needs a game different from or bigger than its existing offerings to really push it beyond people buying a second DS / replacing systems / jumping in late, etc. Same for all other systems, really. Yakuza 3 sold well, but it seems mainly to existing owners. I think Nintendo wanted "General Knowledge Training TV" to be a sort of Brain Training to expand the market, but that didn't go over well, in its first week at least. Seems to be a down week in general.