Managing an infrastructure of peer to peer rental for >500,000 units worldwide seems like a reasonably hard task.

You're comparing apples to oranges here.

Arguably the value of the property Airbnb is a defacto manager for is higher than 500,000 cars, but they also need to expand.

Trust me, I'm totally on board with the unicorn hate here, this seems ludicrous, but I also don't know what is going on at Airbnb, so I try to think about why this might be a good investment, rather than why it might appear ludicrous :)

I'm going to bet they will be opening local offices / retail points around the world. Help less tech savvy folks list on Airbnb as well as serve as a "check in desk" to pickup keys

Cheaper, a simple locker system (nicely AirBnb theme). When renting, you get a code via text, enter it and a small locker opens up with the key. When done, you can deposit the keys in there.

You could even put up a computer next to it where you can search for immediately free rooms, pay and get the key right away, or just one friendly local guide standing around ready to answer tourist question about the city.

Put it in a convenient central location that's safe (airport, central train station).

Unless you have considerable expertise in Airbnb's core and peripheral markets, I don't think it's fair for you to say what is and is not possible for a company to need (putting aside the fact that "need" is used in the normative sense here).