Wouldn't be hard to erase members that have been inactive for so long, might take awhile to run the script... BUT, doing that can create issues with threads displaying... As is the database is already screwed up in the fact you can't search for posts of mine during certain time periods.
So yeah, better to leave the database alone.
Pretty much what I was thinking as well.
Never really advised to hard delete things from a database unless necessary; soft deletes (marking as "inactive") is usually a much better practice for databases.