I've brought this up before, but will (gently) bring it up again.
We just had a guest, who we sympathize with completely, get a 100% day-after-checkin refund from AirBnB because of (completely legitimate) extenuating circumstances. (Massive weather system in the midwest resulted in widespread flight delays and cancellations). We're even the ones who suggested to the distraught guest that they contact AirBnB and use the "extenuating circumstances" reason to ask for a full refund. Because, as I said, we fully understand the guest's unavoidable situation.
That said, we, as the host, end up with $0 for a now-unbookable, lost reservation.
When these situations arise, there are TWO victims -- the guest, who certainly shouldn't be on the hook for the entire original charge, and the host, who likewise shouldn't suffer the loss of the entire original booking (unless they're able to quickly rebook all/most of the lost dates, which is almost always impossible).
What I suggest is that AirBnB come up with some simple formula (for the guest refund) that kicks in a day or two AFTER the original start date. If the host has been able to fill some/all of the original dates, then the guest refund could be based on that. If the host is completely unable to refill the dates, then the guest refund basis would reflect that.
Yeah, there are some complexities here, but some simple rules or guidelines could be worked out. For example, using our latest cancellation (a 3-night booking, which we were completely unable to refill after day 1 was already past), perhaps the guest might've been charged for 1 night out of 3. Or a flat fee, perhaps some percentage of the entire booking or of the first night. Something like that. Basically, something better than exactly $0.
My larger point is that even when truly "extenuating circumstances" occur, there are two parties affected, and two "victims" -- the guests *and* the hosts. Neither should bear the entire burden or reap the entire benefit. That seems so simple and so obvious.