That's very hard to say... After the Smile project was aborted The Beach Boys became "uncool" almost overnight... That definately would not have happened if Smile would've been released. Interesting question... I seriously can't imagine what would've happened. Maybe not much (cause remember, the mainstream public wasn't very impressed by Pet Sounds either), and maybe they would've become the greatest band in music history. Who knows?
I think The Beatles would always have been domineering, because they and the rest of the British Invasion, were very much the catalyst musically and sociologically for, what was happening in the sixties. We don't know how "Smile" would have sounded in 67, but if Brian had gotten it together, it would probably have been one of the very enjoyable classics of that period. I do think it would have had a very important impact on the future of music, much as Pet Sounds has done. But the impact would have been a "long haul" impact, not the immediate onslaught that Pepper induced upon its release. In spite of the popularity of "Good Vibrations", the Beach Boys truly were not perceived by the record-buying public as being pied pipers of cultural transformation like the Beatles were at that time.
But SMiLE would have definitely established Brian Wilson as one of pop music's most visionary geniuses, and the Beach Boys would probably have been admitted into the ranks of "hipdom" as a result, I think.