My experience has been rather the contrary. A couple times when bicycle touring I got to some town only to discover it was "X Festival Week" and the town tourist office could not find a single place to sleep. AirBnB still had places.

My family has had the experience of showing up at a hotel with an advance booking and being turned away because they were full (oops!).

With computerized booking in not sure if that happens a lot. Unless the agent didn't know what they were doing and entered the booking incorrectly a cancelled advanced booking should never happen.

I've also found the larger hotels will keep a few rooms in reserve, just in case there are problems i.e. leaks.

Some (most?) hotels do overbooking, just like airlines.

+1. Had a couple bad experiences. What bothered me more was the complete lack of response by Airbnb on both. Have decided it not worth occasional hassle for what these days doesn't seem to be a big saving unless traveling in a group.

I actually spent a night on the streets in Europe due to AirBnB cancellation - their was no way to get anyone on the phone and they didn't respond to my email.