Nothing.  The Protocol is already quite closed, to the extent that no one knows who controls it.  Which is crazy, if you think about it.  In a lot of ways its already there.  And it could develop in the direction of more open, or more closed.  Nostr protocol becoming more cenralized is probably the biggest threat.

So, HTTP was actually based on SMTP, but with a few more features.  And nostr itself is part of HTTP.  

There are some tools to route round centralization, but it just slows things down.  Not good for nostr, but good for nostr's competitors.  It is what it is.  I fight for an open nostr, you can too!