It's entirely plausible Amazon will grow into its current valuation. What that really means however, is the stock will produce at best a zero return for the next decade. In the case of massive overvaluations, the future gains are merely pulled forward.

If we give Amazon a huge benefit, and say that ten years from now it deserves a 50% higher PE than Walmart currently enjoy (during a huge bull market); spot their future PE at say: 21. They need to generate nearly $8 billion in net income. Once again give them a huge benefit and say they can hit a 5% to 6% net income margin against their sales (Walmart tends to be in the low 3's). They need to get to $160b in sales in ten years, with excellent margins (for what they do). Possible? The sales definitely are, the margin will be hard to hit. That's the good scenario, and if they pull off that good scenario, the stock only breaks even in nominal terms; inflation adjusted it might be down by 1/3.