There are lots of ways to increase crowd size. It's not just bringing in a brand new family who's never visited. It's about getting a one-time visitor to come back a second time. It's about convincing the family who's coming in 2019 to come in 2018 instead. It's about getting an every-four-years visitor to accelerate to every-three years. It's about getting passholders and DVC members to visit more often and spend more time in the parks when they're there.
I think you underestimate the fervor of Disney parks fans. New Fantasyland was intended to do exactly what you're describing. Absorb capacity without drastically increasing demand. They miscalculated and demand was greater than ever, making the most crowded land in the most crowded park even more crowded. Even the freaking quick service restaurant had hour-long lines to get in.
I have no inside knowledge of this, but I think Frozen Ever After represented a shift in thinking in this regard. Rather than try to add capacity the the Magic Kingdom generally and Fantasyland in particular, Disney built an attraction to take some of the Fantasyland crowd and shift them into Epcot's World Showcase. It's the same logic behind Avatar and Rivers of Light, it wasn't about easing capacity issues at Animal Kingdom, it was about giving crowds somewhere else to go in the evenings in addition to the other three parks.
The final piece of evidence in this line of thinking is removal of the nighttime parade at Magic Kingdom. It was controversial, but in my opinion it was clearly an operational decision to reduce crowds by deliberately lowering demand that was otherwise highly concentrated. You can't have that many people who want to be in the exact same place at the exact same time without creating major bottlenecks, which is a guest satisfaction problem at best and a major safety issue at worst.
