Any modern RDBMS is designed to handle millions of records and more, a 50000 record database is hardly gumming up the works. If you consider the size of the user records etc. and make some estimates on how many people fill in user info and how much space that takes up in the database the entire user database file with all user options probably comes to around 25MB. Now, consider the fact that there are 97000 posts on the forums and how much space each of those takes up and you're looking at upwards of 100MB for that database, yet the forum searches are extremely fast so I hardly think there's anything to be concerned about.
If it did become an issue it would be up to Khaled/Tjerk to decide how to fix it, I don't understand why any forum user would be so conerned with what happens to zero-posters' accounts, yet there've been several of these threads brought up that I can recall. Go figure.