That's a given, isn't it? They are nowhere even worth that much today. Private company valuations aren't based on rationality, they are based on emotion.
Uber's current backwards valuation is capturing all speculation about future robot fairies that'll save them in the 5-year future (which always seems to be 5 years out, doesn't it?).
For some reason, the funding geniuses behind Uber seem to assume no other company on the planet can mass produce shared purpose autonomous vehicles at scale? That's just crazy thought. There are four companies today that could entirely wipe out Uber if they wanted to, but other companies are playing a longer game.
Don't compete where the market is, compete where the market is going to be. First mover advantage isn't a thing anymore. First to sustainable scalability combined with emotional grounding is the new advantage.