Yeah but only hardcore fans would do that, the casual gamer isn't going to be able to figure out how to get online with Halo 1.

And yes, multiplayer is the reason the series is as popular as it is now, you can't really debate that. Most people complained that the Halo 2 campaign wasn't that great but tons of people still play it even today. You think if there was no multiplayer anyone would be playing Halo 2 still?