AirBnB's valuation is 25B. It could make a trivial dent in the worldwide hotel market and crush that valuation many times over.
Uber is 50B. The global taxi market alone is worth many multiples of 50B, and that's not counting all the delivery services, bus/public transit-replacements etc that Uber is pushing for.
I'm not saying these companies WILL capture all of their current markets, but it's ridiculous to say the valuations wouldn't be supported if they captured the whole market.
How defensible is the model when then either don't own the real estate (even McDonalds is famous for making money this way) or have contracts to manage the real estate (as many hotels that are now management companies do).
Right now they may seem to own both the people who have places to rent and the people who want to rent but in no way is that as solid as having actual hard assets (or even long term contracts and most importantly a hotel brand). Or even in the case of an Amazon warehousing and logistics (or owning actual data centers).
"For instance, there seems to be a consensus that the most lucrative cab market in the world is in Japan, where yearly revenues are estimated to be about $20 billion to $25 billion just in Tokyo, followed by the United Kingdom with revenues of $14 billion, the bulk from London, and the U.S. with $11 billion overall and about $3 billion in New York. Assuming taxi revenues in the rest of the world add another $50 billion to this total, I arrive at a total market of $100 billion."
http://fivethirtyeight.com/features/uber-isnt-worth-17-billi...
You might as well pretend AirBnB is only trying to capture the hotel market's value. When in fact they're vastly expanding upon it while simultaneously taking market share away from hotels.
Then on top of this you've computed $100 billion per year which can easily justify a $50, $100, $150 billion valuation since you'll in theory be getting that $100 billion every single year