What would have been the effect the novel if Romiette and Julio died like the story Romeo and Juliet?
I think that one effect is that the story would not have been modern. The modern retelling of the Shakespearean drama is one in which individuals possess more autonomy over their sense of self and a greater social tolerance so that death would not have been a ready option. Given how Romiette and Julio were forced into the hazardous situation in the lake, it would have been sadder to see them die in such a manner. Romeo and Juliet made a choice, albeit influenced by society, to take their own lives. Romiette and Julio did not make this decision. They did not choose to be stranded in the middle of the lake. If they were to die, like Romeo and Juliet, it is a bit sadder than the Shakespearean protagonists.
I also think that the more modern context in which Draper places in the characters force the issue that they need to live. There is continual reference to "Romeo and Juliet." This shows that there has been some progression from Shakespearean times. The effect of them dying takes away from this progressive context. If society has evolved since "fair Verona," both kids cannot die. Rather, they face challenge and difficulty, and yet they rise above it. For the most part around the world, social conditions have become more evolved and have matured since the time of the warring households in Verona. Within this, the effect of such a progression is taken away if Romiette and Julio die.