Please do not post thrice in a row, you can just use the edit button.
To answer the questions
World will likely get a reset after each major testing stages (alpha -> beta and beta -> release), or anytime if necessary (happened once already, because there was no iron at all in the ground).
Second, world will be expended if population gets too dense.
Finaly, structures that are not taken care of WILL decay over time, and eventually disapear. For example, buildings that no one cares about that are not inside a claim of any sort (guild or personnal) will decay in a matter of a few days.