Why are we kidding ourselves? Wii Sports is only as popular as it is because it is bundled. Do you think as many people would have spent $49, or even $29 on it if it were a separate game? Sure, some people would have, but not the many millions we're seeing now (well, unless they bundled a peripheral with it, ala Wii Play).
I don't know for sure. When I got my Wii at launch and brought it home over Christmas break to play, demonstrating Wii sports to some of my cousins, aunts, and uncles was the very reason they went out to get one for Christmas that year (or shortly after if they had trouble finding it). I think 4 or 5 of my family members purchased a Wii because of that. I really do believe that they would have also bought Wii Sports if it was not included because they were so enthralled with the title (well, maybe except for one of my aunts who only bought a Wii to play Twilight Princess - she's obsessed).
Now, the question is, would I have bought Wii Sports at launch if it wasn't bundled, which in turn persuaded my family to buy Wii's for themselves? Probably not...