Why one analyst thinks Tesla's Model Y could 'ultimately cannibalize' some Model 3 sales (TSLA)

Tesla is due to begin deliveries of its Model Y vehicle in March 2020. 
"A key question is whether Model Y demand will ultimately cannibalize Model 3 volumes," analyst Toni Sacconaghi of Bernstein wrote in a note Tuesday. 
He added that any softness in Model 3 sales could spook investors near-to-medium term. 
Here's why Sacconaghi thinks the Model Y could take sales from the Model 3. 
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