China, biggest nation of the world, also one of the least religious...
Are they though? There is a lot of ancestral worship and the following of ancient Chinese spiritual beliefs going on in China, isn't there? Isn't that pretty much "religion"? Does a religion have to be organized the way the major world religions are?
Personally I don't think religion will ever die out. It's been pretty much with us from the days when early human tribes started having a shaman or a similar figure talk to the Gods to try to predict the weather or the next harvest.. and probably well before that even. Religious thoughts were likely with the very early humans, who could not comprehend what the motorboat was going on around them, what from the earthquakes, volcano eruptions, eclipses, floods, the changing of the seasons, the miracle of birth.. I think that as soon as humans could reason in a complex enough manner to try to explain why and how things were happening, that they likely started attributing a lot of what was around them to the supernatural, gods and spirits who orchestrate such events on this world. That's pretty much religion.
Humans are superstitious, I don't think we will ever lose that attribute of who we are. Not all of us are superstitious, but many of us are, and I think that even in the year 3,500, 1450 years from now, when we have flying boats and when America finally gets universal healthcare, there will exist humans who believe in things that are outside of science, people who believe that there are supernatural agents out there influencing our world, people who pray to saints, their ancestors, gods, angels, and other non-worldly beings, people who believe in ghosts, miracles, and other things not proven by science. Even if there is no organized religion the way it exists today, why wouldn't there still be people around who harbour some of those beliefs? Science will never explain everything. Isn't that something Godel proved at some point?
While religion is on the decline in the developed world, and which points to a broader dynamic of people in developed countries gradually becoming less religious over time, I don't think this will ever eliminate all religious or spiritual thought. Even if scientists figure out how the universe was created and all the mechanisms associated with that, there will always be something that some people feel requires a supernatural explanation. Science never proves negatives, so it will never prove the absence of an afterlife, for instance. Even if it turns out that the afterlife exists and that scientists figure out how it works, I doubt all humans will accept that this is basically it, as far as the afterlife goes. Some people will continue having their own personal or organized religious beliefs about the afterlife, the same way a lot of people today hold alternate religious hypotheses about the origins of humans on this planet, dismissing the already figured out theory of evolution, even in semi-developed places like Florida. Even the best developed parts of the planet, with the best quality of life and education, like Norway and Switzerland, contain significant groups of people who hold religious beliefs. Even if all organized religion shut down in those countries, those people would continue to exist, likely meeting and praying together, and discussing religious matters in their homes and in community centres, even if churches and priests no longer exist.
Retro is also always in, it seems. Even in the scenario of all all organized religion disappearing off the face of the planet and the solar system, some people will dabble in ancient religious thought, the same way there's dedicated groups of humans today who dabble in retro gaming, retro music, and even attend retro medieval-themed renaissance fairs, just to name a couple examples.
For all these reasons I don't think religion is ever going away.