actually seasons are not working that way only.
seasons in Diablo 3 allow you to move everything you got until the season ends to your main account after it expired, and allows to get new items/sets earlier than on vanilla game. so its not just to be unique items every time but instant development and balancing: every single seasons there are a lot of new item sets are being developed for the next one. no one here has time/motivation to act this way, devlopment community in Rappelz is unstable (well, yet unstable).
without the other side you didnt mention, seasons or anything else that even closely looks like seasons, doesnt make sense since Rappelz players audience usually contains people who don't even take a look on a server that will even try to tell them "you will lose something". not talking about every player round there but about the most.
the point of making servers for month or a couple is to collect donations and disappear since there is nothing to donate for already in case of limitation of a total audience.
but there are a lot of other things that make servers dying, most of that - unhandled "accidents" that make owners just give up because they have not enough experience or motivation to continue to serve a project, as like: attacks, black hype, broken economics, contents race, etc.
this idea is very good but the main idea implementation from Blizzard contains much more pros than it looks like and if it will be implemented in Rappelz (and that is surely possible) - it would make a game more interesting, maybe, but will not solve a problem you talk about.
I have another idea to implement that is actually close to Diablo 3 game mechanics but i won't tell a **** yet haha :P P.S. Blizzard rocks