Ah.
I was thinking unused ones PMs delete after a while. I mean if nobody checks it in a month, it prolly wasn't important. Inactive accounts should too. We have 22800 users but really only a fraction of that has signed on this year.
Think about it this way... even if space is at a premium, the server has no problem making room for every post everyone has ever made. If I can make 5,000 posts and not tax its capacity (not to mention responsiveness), why can't I make 200+ private messages? PMs are just another type of post, and posts have no limit.