.. and what guarantee do we have that there will be radiation in the future?
There's nothing said about "post apocalyptic" themes in the game description. The current map is a TEST map. The final will be VERY different. Zombies were a test. Military weapons will be removed. So how do we know that radiation wasn't a test? What proof do we have that that is a definite thing?
As I said in a previous post. For all we know, the final map might just be different regions, climates and terrains. It might be a completely blank canvas (I hope it is).
One thing that support Rust being some sort of "Post-Apocalyptic" area is The game's logo/icon

Doesn't it look similar to something??

Obviously the Rust icon is a Radiation symbol which supports the idea of Rust being a "Post-Apocalyptic" game