I'll have to necrobump and update this thread with sad info.
My prediction sucks and it was wrong all along.
Since VGC hasn't updated the numbers, one sweep at Nintendo's Software Sales Unit data tells us all that we need to know.
Mario Kart for WiiU is at 5.87 total sales.
If VGC is correct, then we have that the lowest-selling game was Super Circuit:
7 | Mario Kart: Super Circuit | GBA | 2001 | Racing | Nintendo | 2.62 | 1.64 | 0.99 | 0.23 | 5.47 |
MK for WiiU beat it. Hence, it's no longer the worst-selling Mario Kart.