When they imposed that charge a few years ago not only were bookings down, but fares and the stock price was slumping. This year bookings and prices are going up and up...at least for cruises in North America. Only about 30% of Americans have a passport. Close Loop cruises within North America are the ONLY way for 70% of Americans to travel outside the US. You can sail on a 7 day Caribbean cruise with a birth certificate and state ID but you can not drive in to Canada or Mexico for the day. As long as the Closed Loop Loophole remains open, cruiselines will do well in the Caribbean.
So I agree that they will offset costs with amenities or nickle and dime you onboard. Three years ago that wasn't possible for them.