I mostly agree with you. I have found full-service long-term property managers that do everything for 12%. They charge owners for cleaning/maintenance/repairs, but they still manage it all.
As far as I know, Airbnb’s average fees are 15% and their minimum is 12%. They are not transparent about their guest service fees, but even if the fees are only 12% for long-term rentals, that is high for the services they provide for long-term rentals, because Airbnb doesn’t provide specific support for long-term rentals at all. In fact their policies increase the owner’s risks for long-term rentals.
The real question is: Is the value of Airbnb’s marketing is worth the extra cost? I.e. can a host get a higher price for a listing because Airbnb’s marketing increases the demand for the listing? Is there evidence of this for long-term rentals, and even if there is, does it outweigh the added risks?