Hi!
This is certainly a valid concern and is why Airbnb highly recommends that guests and hosts be in contact via messaging before the guest even arrives 🙂 That way you know the host is responsive. Airbnb actually does NOT recommend you stay at a place where the host has been non-responsive, even if the reviews are good. Good reviews DO say a lot though, so it's a great sign your host has good reviews. It's better than if you're doing a host with 0 reviews or 1 review (who could possibly be a friend, lol)
I am not sure if you've hosted or if you just travel, so as a host, let me tell you something! Airbnb actually rates us by our response time. (Sadly, "accepting" a request, even if you never message back, counts as responding.) Airbnb also rates us by if we CANCEL an already-existing reservation. We will potentially lose SUPERHOST status and also possibly get bumped further down the listings, so most hosts will rarely cancel on you. It reflects badly on us.
Most hosts would rather decline if there's a potential clash with you staying, than EVER cancel, because canceling is bad for everyone- airbnb, the guest, and the host.
The tricky thing is, if someone did get cancelled on, they wouldn't be able to leave a review saying that happened, since airbnb only allows reviews for completed stays (as far as I know, somebody correct me if I'm wrong!)
If you are VERY concerned, I suggest:
a) ONLY picking Superhosts, which are people who have completed more than 10 guest stays, have a 80% or better response time, and do not cancel on their guests. It'll say on people's listings if they are superhosts and superhosts generally show up first on the listings.
b) Getting in touch with the host and having them respond to you, BEFORE your stay. That's a good sign they won't cancel.
c) If all else fails and the host does cancel, yes, AirBNB can help you.
AirBNB will put a hold on the funds so that the host can't get paid. They'll refund you ALL of your money if the stay falls through. Go to the AirBNB contact page to submit a complaint or request for help. However, understand that this refund does NOT work after the first day so you need to contact AirBNB ASAP. AirBNB CAN choose to give you a credit, but it's totally up to them. I'd guess they may say no, since you'd be getting all your money back anyway. BUT it may be possible to get a credit towards another airbnb place, especially if there were NO other similar-price options and this host totally ruined your plans. Again, all up to AirBNB.
I hope this helped! 🙂 Good luck on your travels.