You have been posting those announcements for a few years and now old event announcements are useless to most people. These posts are still an asset to your website. They still attract traffic and they still have some nice links. They might produce nice income because the people who visit them see the stale date and are easily distracted by ads.
At some point these old event announcements should be retired. You hate to throw away the income and content and labor but there might be a nice gain - redirect those pages to the homepage of your events section. The redirected links that hit those old pages might transfer a little power to your main events page. Perhaps that will get you higher rankings for a couple terms that you have not yet acquired #1 rankings.
It's possible that a smaller number of visitors into relevant pages will be better for the health of your business than a larger number of visitors into topic-defunct pages. I think that a smaller site will rank higher if the linkvalue transfers well.
Have you ever considered closing part of your site? Have you done this before and seen good results? Although my example applies to events this could also apply to retail, forums or other types of sites.
Edited by EGOL, 02 February 2009 - 05:04 PM.