itch.io

Free versions will stay free, paid versions will stay paid. That being said I think the upcoming 0.18 will be free, and maybe in the far future another update will be free. But 0.18 may be the last free update to be released. 

Edit: I didn't think on the potential of what you said, I may in the future release older version for free, but if I do that I'll eventually only go to half the game. After that I'll include a very simple demo once the full game is released.