> Can anyone guarantee those things?

No one can guarantee those things.

No one can guarantee anything in this world.

You are free to choose non existence but others are equally free to be brave enough to wake up in a worse world.

They may even feel responsible enough to try and fix it rather than requiring a "guarantee".

> The assumption being that, this "new world" is a better place than where you are now.

No one is assuming that. At least, I am not assuming that. Even if the world gets worse, I think it is rational to want to live longer and try and fix that.

Even if it is provably 100% unfixable and worse, any existence is better than non existence (certain forms of Hindu/Buddhist meditation teach you how to get into a state where that is obvious).