"If my grandmother had wheels she would have been a bike" is a bowdlerization of a much older phrase. As my Great-Grandfather, born in 1890 in Widz, Belaruz would say it "ווען די באָבע וואָלט געהאַט בײצִים, װאָלט זי געװען מײַן זײדע" (If my grandmother had balls, she would have been my grandfather.)

In your understanding, what is the point of the original phrase?