Assuming the tech is in place and there are many otherwise equal firms entering the market, Uber would have a competitive advantage from their pre-existing customer base. Plus already having the tech in place for arranging the trip. Does that justify their valuation? Maybe, maybe not. Even if some other firm beats them to market with driverless tech, they can always buy driverless cars from that company, or that company may buy Uber just for access to the ready made market. But it's all very theoretical and far off, so I'd say it probably doesn't justify the valuation. Their ability to pivot to actually profiting off their current services is what probably matters most to their near term valuation.