It all depends on chronology. I think that you are missing the temporal element of when the fees switch over to being included in the resort fee. Assume that Cactus Resort and Spa charged $400 per night for its room, and charged $20/hour for tennis court time, and $10 to use the sauna, and $3 for the bottle of water on the desk in the room. Then one day the resort decides to make a change. It decides to increase its room rate to $425 per night, and now includes an hour of tennis, the use of the sauna and a bottle of water. In that case, yes, you could say that the costs are "built in to the cost of the room." But instead of doing what it did, Cactus Resort and Spa could keep its rate at $400 and add a $25 per day resort fee which includes the tennis, sauna and water. In that case, it cannot be said that the $400 already includes those items. Instead, those items are now part of the resort fee. They were never free back before the change. They were cash-based. One can't argue that $400 is a lot to pay for a room and therefore all of those items should have been included from the get-go. They were always added on extras. So when the resort decides to make a change, either the room rate has to go up, or a resort fee has to be assessed. But things that were not free before are not suddenly going to become free.
Again, this example applies to the proper way that a resort fee is assessed. If Disney assesses one and does not fold into that fee things that people are currently paying for, then it would be abusing the concept of what a resort fee is.