You yourself said.... "Some People". I believe it was you who posted the endurance of most charaters...which would seem to preclude any need for forced expiration.

Naturally limiting slots as well as creation windows would undoubtably give you the same results without seeming so totalitarian. I proposed an alternative.

The seasonal/limited races are a great idea...made bleh..

Might I suggest that you give consideration to some food chain Marketing examples. Pizza hut seasonal specialty pizza's come to mind. When sales dip, they roll out limited time special foods. this peaks interest and increases sales. But they dont come to your table and take your uneaten pizza if the offer expires.

Seasonal races = Awesome.
Forced expiration = Terrible for many reasons.