How/why did Tomorrowland depopulate?

The movie itself does not adequately describe why Tomorrowland is depopulated and left in decay. Many things are explained in the prequel novel titled Before Tomorrowland.

The novel, Before Tomorrowland described by author Jeff Jensen:

“It’s a distant prequel to the movie,” says Jensen, who co-wrote the book with artist Jonathan Case. “You will not meet any characters from the movie in this story, but you will get to know in a pretty deep way the organization that’s responsible for Tomorrowland.”

There’s a whole 120-year history of the secret organization that built this futuristic realm, and one of Jensen’s duties on the film was to construct the backstory of this group of inventors, geniuses, and dreamers. It’s the foundation upon which Tomorrowland the place and Tomorrowland the movie rests.

“The city was the grand gesture of this organization,” says Jensen, who worked on the novel with major story input from director Brad Bird and screenwriter Damon Lindelof. “It wasn’t the goal of the group from the start, but over time it became the goal: A living laboratory where things are always changing and the best ideas are either given to the world or saved for a time when the world can handle it.”

The novel describes Tomorrowland as an entire planet in itself. The planet was discovered by a group called Plus Ultra or +U right before the 20th century. Nikola Tesla, a member of +U invented a transport mechanism called The Grid which is how +U members traveled to Tomorrowland for colonization.

The novel gives more of an insight to why Tomorrowland is depopulated than the movie:

The Tomorrowland we see in present day times seems very depopulated, and in significant disrepair. My guess is that in the face of Nix’s authoritarian (and increasingly militaristic) leadership, many members of Plus Ultra were either forced out, or chose to leave.

Governor Nix (portrayed by Hugh Laurie in the movie), the leader of Tomorrowland did not want anyone from Earth to know of its existence. He believes that other humans from Earth will pollute and corrupt the planet. Nix basically wants Tomorrowland to be left alone, believing that Earth will destroy itself.

I saw Tomorrowland yesterday and this is what I understood:

• When Casey Newton is discussing with Frank Walker about Tomorrowland he told her that what he saw was just a presentation, something like a demo.
• When Frank Walker arrives for the first time in Tomorrowland we don't see all the people that Casey Newton sees.
• The robots at the store, and Athena say that people were suppose to be invited/recruted in Tomorrowland but they never were.
• We know that Frank Walker was banished from Tomorrowland.
• We can't actually distinguish the people from the robots (like Athena).

So I came up with two theories:

1. All that Casey Newton saw was fake, an intelligent presentation of Tomorrowland. There were just the people that built the place and we can assume that there were not that many and they died.
2. The people were sent back like Frank Walker.

Tomorrowland is depopulated all because of the monitor that Frank invented. Once Nix saw that the world was going to end he gave up on humanity. The whole purpose of Plus Ultra is to change the world for the better. There was no point in improving the world anymore because he knew the world was going to end. Because of that he threw out all the recruiters.