Any argument that claims Halo 3 is niche is claiming video games are niche, simple as that. FPS games sell a ton, and I know people from 5 years old to 50 that play them. It is easily the most popular genre in gaming right now. Platforming is not even close to as popular, and 2D gaming is even less popular than that. If anything NSMBWii should be considered niche in the context of the video gaming world, and we already know a similar game wouldn't do nearly as well without the Mario name. But neither are. If a game can sell more than 1-2 million, it's not niche. If it can sell more than 10 million it's really, really not niche. Love simulators are niche, platformers and FPSs are mainstream.
You do not have the right to never be offended.