I think if SMiLE was finished and released, it would've been successful in reigniting The Beach Boys' popularity, but in a completely different way to it was when they were singing about surfing/cars/summer. They would've been appreciated as part of the hip psychedelic scene alongside The Beatles, early Pink Floyd, the Laurel Canyon artists/bands etc. and their next few albums would be much higher anticipated than they were in reality.
I agree Brian probably would've followed up SMiLE with something similar to Wild Honey, although Mama Says most likely wouldn't have been on it. Friends would remain mostly unchanged as well, apart from perhaps the Child is The Father of The Man section from Little Bird. It does make me wonder what the SMiLE songs on 20/20, Sunflower and Surf's Up in this universe would be replaced with.
Heres how I would have done it
Wild Honey: Been Way Too Long
Friends: Same (Would be a nice call back)
20/20: We're Together Again and Ol' Man River
Sunflower: Same
Surf's Up: Retitle the album to Landlocked and make (Wouldn't It Be Nice) To Live Again as the closer