Nothing. Thrive will be free and open-source forever.
You will control many cities forming your entire civilisation, similar to how most grand strategy games work. I think @NickTheNick is more qualified to give a detailed answer here.
On the hardcore settings, yes - if when setting up a new game you specifically choose this to be a possibility, there will be a small chance of random game-ending disasters, such as gamma ray bursts or asteroid impacts, ending your game without warning. Why anyone would want to play like this I don’t know, but apparently some people do.