Uber is worth $17 Billion because that is how much someone paid for it.* The question as to Uber will ultimately be worth $17 Billion when those investors try

Uber is worth $17 Billion because that is how much someone paid for it.* The question as to Uber will ultimately be worth $17 Billion when those investors try to sell their shares is another question. And to that question, the Aaron Levie tweet is a good heuristic - "Sizing the market for a disruptor based on an incumbent's market is like sizing the car industry off how many horses there were in 1910." *of course they only bought part of it, but the easy math of 'they bought x amount for y, so total worth is z'