Will the full version ever be free?
Probably not :(
I believe the author has said before that they don't intend on releasing the full version for free, although take my words with a grain of salt.
However! As you may already know, us demo players do get monthly (it might be once every two months) planet-swaps, letting us experience a taste of the paid version! They have said before that these planet swaps will continue post-release as well.
I'm not trying to answer on the author's behalf, but I thought I might as well reply anyway. Hope that answers your question! :]