That's not the whole story. It's not just more subscribers, it's that more of the subscribers are downloading as opposed to streaming, and as people get more internet TV boxes and faster connections, more people are streaming and they're streaming more movies per month. When they started streaming, a big reason they got the content so cheap was because studios thought nobody would watch it.
I'm sure the studios don't want to put NF out of business, but they don't want to let NF cannibalize other (per view) streams of income either. People seem to assume that the current system will go on indefinitely or even get better, but there's no guarantee of that.
It just so happens there's an article that agrees with my earlier post, NF's contracts are coming up soon and the studios are not happy with them or likely to renew on the same terms.
http://blog.moviefone.com/2011/03/15/netflix-future-contracts/?_r=true
http://www.thewrap.com/movies/blog-post/netflixs-brick-wall-25501?page=0,0
And bandwidth caps are a whole other can of worms with the potential to put a huge dent in NF.