Feedbin is not an archival tool and does not have the goal to store all the RSS on the Internet.
The main considerations are limiting database size and maintaining good performance. Storing more stuff costs more and makes performance worse.
Feedbin stores 400 of the most recent articles per feed. Feeds with more that 400 articles are truncated daily to keep them from growing unbounded.
There are no limits on the number or the age of unread items you can have.