"Do Androids Dream of Electric Sheep?", a 1968 book, was set in 1992, and featured sentient androids hardly distinguishable from humans and off-world colonies i

"Do Androids Dream of Electric Sheep?", a 1968 book, was set in 1992, and featured sentient androids hardly distinguishable from humans and off-world colonies in (presumably) different star systems. So yeah, 20 years to build a space elevator is, indeed, a very conservative estimate by the standards of 1960-1990s.