I don't think either of them would have been succesful individually. Smile was a dead-end because none of the three members had a real singing talent, and Freddie needed a better backing band.
The difference between us of course being that I don't see Brian and Roger as a "backing band", but as an integral part of the band (just like Freddie). No-one could really know back then that Freddie would be the great rock vocalist he would become. He had the image, but not yet the fully fledged voice. I think Brian and Roger thought of their union as a little bit more risky than Freddie did.
I would agree with something I've read... That drums and bass are working as istruments if you DON'T pay attention to them. They're forming a base obviously, but they aren't really meant to be listened. :/
In some way I can see what you mean, but that doesn't mean they can't be listened to. And even if you think of them as "only" the foundation (which they are), a foundation can be built in many, unique ways. If you swap Roger with some other drummer, it won't be the same (the same goes for John, of course).
Brian's guitar work is fine if it's limited to less than ten seconds at a time. Then again, I don't really like any guitar solos.
Ten seconds? Hmm... I don't see how you can be a Queen fan, and not like Brian's solos. But maybe you are not; you might be a Freddie fan, which is a different thing. I'm beginning to suspect that... :)
And, besides, Brian's guitar playing isn't just about solos, it's about everything else he does, as well.
But, to give this another angle, Freddie's piano playing was also a huge part of Queen's sound, which is often overlooked. Both Freddie and Brian were fine pianists, but their playing styles are very different from eachother... But, let me guess, you haven't noticed that! :P