Step 1. AirBnB, VRBO and others list inventory for free. Step 2. Google starts offering rentals straight from search and forgets you have a site. Remember Facebook News anyone?
Exactly this! Google Flights is already a thing and the hotel search works great too. Once they have enough people directly searching from Google, they can either charge Airbnb or hotels to send traffic or build its own Airbnb backend and roll it as Google product.
Google is doing this with everything. In the end they will answer almost every query directly, except when you are looking for a site. Even then, they could try to identify what do you want from that site (if you use chrome or the site has google analytics) and show you the info you are lloking for.