I don't think their base of potential customers is tapped out at all. But any significant increase is going to cost them.
Could cost them in revenue by dropping prices or giving "free" dining away. Could cost them significant money in new attractions and improved maintenance. I'd float increased staffing but I'm not sure they could boost cast numbers significantly if they wanted to.
There are levers to pull, but I don't think they want to pull them until D+ is profitable and they can take a little blowback in operating costs. And obviously Bob the former would prefer never to pull them. He just won't have a choice soon.