Fewer and fewer hotel reservations let you cancel until 6pm day of though like they used to. More and more cancelable reservations want 24, 48, or even 72 hours these days.
To me a planb is to have a few hotels, bnbs in ideal locations i can book at reasonable short notice. I know they have space and they are in my budget. I have extra cash so I don’t have to wait for a airbnb refund if they take time. I don’t use other airbnbs as a planb in case something happens with airbnb. A couple of years back we booked a lovely place in normandy, france with indoor pool for feb half term. The owner cancelled while we were on the ferry. Luckily i had a plan b we drove to a lovely chateau near mon st michel and enjoyed our stay. Airbnb did refunded me quickly but not in time to depend on it for checkin at the chateau. I booked a flat in the uk for a colleague to stay while in uk for a few weeks. The host tried to bait and switch i refused and found an alternative airbnb which as it was went ok. I had to wait for a refund because i paid for 3 weeks.