Fundamentally, we Atheists would love to share our secret. We know how nonsensical religions are. We feel a collective freedom. We feel it's a better way to go through life, and we'd love for others to join us.
But what if this actually does happen. Atheism catches on, and 15% becomes 50%. Would this be a good thing? Maybe, maybe not. I imagine a scenario where the remaining believers become even more dug in, more fanatical. It almost has to be that way. In order to continue to believe in what people are running away from in droves you would likely have to become more extreme in your beliefs.
I imagine that in said scenario, we Atheists will become even more dug in ourselves, so convinced of our correctness based on the throng of converts that any remaining believers are surely ignorant fools.
What we have here would be a truly divisive rift. Two polar oppositve extemes trying to coexist in a world/country with no buffer. It's a recipe for disaster. Civil wars begin in such ways.
I admit that it is an extremely unlikely scenario. But the possibility of such an occurrence does beg the question - should we just keep our secret to ourselves?