If HAL didn't make more net revenue from those "free" casino people they wouldn't give away the free rooms. Obviously they make more up front from cash paying customers but I suspect the bean counters know that gamblers lose, on average, more than the price of the rooms and lose more when the rooms are "free" than they would if they were paying.
As for people using FCC, I suspect many of them are unpleasantly surprised to see how expensive some cruises have gotten, especially in the Veranda cabins and above. Bargains can still be had on cheaper cabins but they are fewer and further between. So, the choice to take 150% FCC in 2020 might not have worked out much better than taking cash since the higher prices are eating up a lot of that bonus credit.