I tottally agree with this feature. I'm playing WURM atm, with loads of abandoned places, buildings, fences... Imagine a new player looking for a place to settle... nightmare. If decay is implemented, they can damage and turn into ruins, making it a lair for some vultures, animals or even zombies.
Decaying sounds good, but it has several problems as well. The main one is that it takes really long...why keep up space for 1-2 months if the player has left the game already ? The other problem that it will not encourage the old player to return to the game....why would he, if everything he has worked for disappeared.
So personally I think it would be better to kind of pack up the quitting player's property, and move it into his bank or any type of storage. When he cancels his subscription all his properties (houses, mines, whatever) should disappear from the game, and the materials he used to build them should appear in his bank.