I wonder how much of this is the recently enforced payment in your credit card's primary currency (with the ~3% spread going to Airbnb as profit).

Due to this - and Airbnb's refusal to even discuss their reasons for it - I always consider other options before Airbnb. In the past it was the other way around.

Yep, this was an incredibly frustrating cash grab. I already have a credit card that doesn’t have international fees and provides decent currency conversion rates. I also started looking elsewhere first.

How would gaining a 3% spread lead to 50% revenue increases?

My guess is assuming Airbnb takes 10% of the total booking cost - the additional 3% spread would mean a 43% increase in revenue

That's ludicrous. Even the mom and pop hostel in Tokyo that charged me ~$20/night asked me which currency I would like to be charged in.

AirBnB popularity has gone through the roof in many places. If you have visited Rome or Venice, it the defacto place to look for accommodations. You can’t walk in Venice more than half hour before you see a tourist dragging thier luggage on cobblestones street to their AirBnB.