Seriously doubt it. I don't know a single person who bought the first two Halo games based on visuals, I do however know a ton who bought Gears based on that.
You either don't know very many Halo players or you're the only one you know of who bothered to play the first two.
I didn't bother paying attention to the PR of the second game because I was still pretty angry that I spent money on the first one. I, and millions of other Halo consumers, had seen the mass exhibition of graphics by the Bungie team. Aside from pointing out the innovative nature of killing aliens on a huge leafy space-bracelet and flying purple oblongs, there is simply nothing else to the game except for your usual first-person stuff. This wouldn't have been so bad if the story weren't so abysmal.
At the time, I , and many others, got it for the graphics.
When other games surpassed it, I looked to gameplay and failed to find anything unique.
Then I tried enjoying the story and....Yeah.
In reality, the only thing that's put this game on such a high pedestal is that it became XBL's first crown jewel. After that, there was no point to try and present anything better, because they had something stable and loyal fanbase that found a niche outside of the kiddy-centered Nintendo and the none-internet associated Playstation.
In the end, it really doesn't matter if you can prove they did or di not get it for graphics. The point is that Halo has a loyal fanbase attached to it, and even if Halo 3 did indisputably suck in every facet (although, IMO it already does), they're not going to let go of it. Ever.