Unlike demo, you get to keep the prologue forever (since it technically is a complete product). While dev can disable the demo so you won't have access anymore, even if the dev disable the prologue from sale, players who acquired it will still can play, and download it whenever they want. Steam treats all purchases, free or not, with the same level of seriousness. Fun fact: I had to rewrite the description for the prologue because Steam wanted to make sure "players know what they'll get with their purchase".