That two billion budget isn't just one film, it's four films, alternate media, the investment to create technology and methods in filming, post production, CGI advancement, retooling and re-release of the first film, marketing and advertising, in park attractions.... I've mentioned before it's an investment that will pay off over time VS the initial film runs.
Until a term is coined I'll call it "multi generational investment"
Someone was wise enough to realize nostalgia affects people and a method to separate the nostalgic person from their money. People not born yet will see the third or fourth Avatar film in a theater, then want to see the first one, then want their kids to do the same... Of course we're skipping the current generation as that already exists.