More likely this is a version of the "Eternal September" problem. Early adopters of Airbnb were more likely to be "true believers" of the idealized vision of a kind/generous host matched with adventurous/courteous guests, but this vision gets diluted over time as the userbase grows. Once Airbnb reached true ubiquity, the general user population is just looking for a good deal and isn't nearly as savvy or as committed to the original ideals; this applies to both hosts and guests, of course.
It's impossible to scale a platform to 10s/100s/1000s millions without finding an ever-lowering lowest common denominator.