I notice that many powerful blogs when they post will gain rankings for as long as the specific post remains on the home page. Constantly changing content will lead to constantly changing rankings. While blogs do archive content, by nature, they devalue content with age as the more powerful PageRank and backlinks are generally coming from the main blog page.
There is a simple fix for this that is optimizing the individual pages with links. Not all posts will continue to gain internal links as they age, so sometimes building links can help pick those pages back up. Adjusting the sidebar to link to specific posts will help those posts as well, but when we talk about hundreds of thousands of pages, it is likely the rankings will fade over time.
In most cases, time and the devaluation of a post will push the rankings down as competitors become more powerful they will again outrank the blog. In developing sites as blogs keeping this in consideration during development can help keep rankings more consistent for long term rankings that static sites generally see.